branch-protection: declarative policy + apply script (#89)

Branch protection on main, declared as code rather than as opaque
GitHub UI state. Pairs with the CODEOWNERS chassis (#88): once this
PR lands and an admin runs the apply script, every PR to main must
satisfy code-owner review and the listed required checks.

Components:

- .github/branch-protection.json — the policy. Edit this to change
  required checks, review counts, etc. Includes a _comment field for
  human readers; the apply script strips it before PUT.
- scripts/apply-branch-protection.sh — idempotent apply via `gh api`.
  Reads back current state for verification. Supports DRY_RUN=1.
- docs/branch-protection.md — explains the policy, how to apply, how
  to change, why declared as code.
- AGENTS.md topic-index row.

Policy summary:

- Required status checks (strict): Classify Changes, Check AGENTS.md
  Links, Test Workspace, Test omnigraph-server --features aws,
  CODEOWNERS / drift, CODEOWNERS / noedit.
- Required approving reviews: 1, must be a code owner.
- Dismiss stale reviews on new commits.
- Required linear history (squash or rebase merges only).
- No force pushes, no deletions, no admin bypasses.
- Required conversation resolution.

What's NOT in this PR:

- Required signed commits — not yet; maintainers must enroll GPG/SSH
  signing first or merges will block.
- Tag protection for v* tags — separate PR.
- Additional required checks (cargo deny, audit, fmt, clippy, CodeQL,
  schema-lint MR-946) — separate PRs as each lands.
- The script is NOT run by CI. Branch-protection changes are admin
  actions; CI-driven auto-apply would defeat the purpose. Manual
  invocation is the audit point.

How to apply after merge:

  ./scripts/apply-branch-protection.sh

Requires gh-CLI auth with repo-admin permissions.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.github/branch-protection.json

@@ -0,0 +1,30 @@
+{
+  "_comment": "Branch protection policy for main. Applied via scripts/apply-branch-protection.sh. See docs/branch-protection.md for rationale.",
+  "required_status_checks": {
+    "strict": true,
+    "contexts": [
+      "Classify Changes",
+      "Check AGENTS.md Links",
+      "Test Workspace",
+      "Test omnigraph-server --features aws",
+      "CODEOWNERS / drift",
+      "CODEOWNERS / noedit"
+    ]
+  },
+  "enforce_admins": true,
+  "required_pull_request_reviews": {
+    "dismissal_restrictions": {},
+    "dismiss_stale_reviews": true,
+    "require_code_owner_reviews": true,
+    "required_approving_review_count": 1,
+    "require_last_push_approval": false
+  },
+  "restrictions": null,
+  "required_linear_history": true,
+  "allow_force_pushes": false,
+  "allow_deletions": false,
+  "block_creations": false,
+  "required_conversation_resolution": true,
+  "lock_branch": false,
+  "allow_fork_syncing": false
+}

AGENTS.md

@@ -94,6 +94,7 @@ Full diagram and concurrency model: [docs/architecture.md](docs/architecture.md)
 | Deployment (binary / container / RustFS bootstrap / auth / build variants) | [docs/deployment.md](docs/deployment.md) |
 | CI / release workflows | [docs/ci.md](docs/ci.md) |
 | Code ownership (CODEOWNERS source of truth, roles, regeneration) | [docs/codeowners.md](docs/codeowners.md) |
+| Branch protection policy (declarative, applied via `scripts/apply-branch-protection.sh`) | [docs/branch-protection.md](docs/branch-protection.md) |
 | Constants & tunables cheat sheet | [docs/constants.md](docs/constants.md) |
 | Per-version release notes | [docs/releases/](docs/releases/) |
 

docs/branch-protection.md

@@ -0,0 +1,83 @@
+# Branch protection on `main`
+
+`main` is gated by a declarative branch-protection policy. The source of truth is `.github/branch-protection.json`; the apply mechanism is `scripts/apply-branch-protection.sh`. Re-running the script with a changed JSON is idempotent.
+
+This page explains what the policy says and how to change it.
+
+## Current policy
+
+| Setting | Value | Why |
+|---|---|---|
+| **Required status checks (strict)** | `Classify Changes`, `Check AGENTS.md Links`, `Test Workspace`, `Test omnigraph-server --features aws`, `CODEOWNERS / drift`, `CODEOWNERS / noedit` | Every PR must pass workspace tests, AGENTS.md link integrity, and the CODEOWNERS hygiene checks. `strict: true` requires the branch to be up-to-date with `main` before merge. |
+| **Required approving reviews** | `1` | At least one reviewer. With a 2-person team, going higher would block all merges when one person is unavailable. |
+| **Require code-owner reviews** | `true` | The reviewer must be a code owner per `.github/CODEOWNERS`. This is what makes the codeowners chassis enforced. |
+| **Dismiss stale reviews on new commits** | `true` | A push after approval invalidates the prior review. Prevents the "approve, then sneak in unreviewed changes" pattern. |
+| **Require linear history** | `true` | No merge commits — squash or rebase only. Matches recent practice. |
+| **Disallow force pushes** | `true` | No history rewrites on `main`. |
+| **Disallow branch deletions** | `true` | `main` cannot be deleted. |
+| **Required conversation resolution** | `true` | All review comment threads must be resolved before merge. |
+| **Enforce on admins** | `true` | Even repo admins go through the gates. The point is no bypasses. |
+| **Required signed commits** | not yet | Not enabled. Would lock out maintainers until everyone enrolls GPG/SSH commit signing. Tracked as a follow-up. |
+
+## How to apply
+
+Run from the repo root:
+
+```bash
+./scripts/apply-branch-protection.sh
+```
+
+The script reads `.github/branch-protection.json`, strips the human-readable `_comment` field (the GitHub API rejects unknown keys), and PUTs to `repos/ModernRelay/omnigraph/branches/main/protection`.
+
+Requires `gh` authenticated with a token that has admin permissions on the repo.
+
+To preview without applying:
+
+```bash
+DRY_RUN=1 ./scripts/apply-branch-protection.sh
+```
+
+## How to change the policy
+
+1. Edit `.github/branch-protection.json`.
+2. Open a PR. The JSON change goes through normal review.
+3. After the PR merges, an admin runs `./scripts/apply-branch-protection.sh` to push the new policy to GitHub.
+
+The script is **not run automatically** by CI. Branch-protection changes are admin actions that should be applied deliberately — a CI-driven automatic apply would mean any merged PR could rewrite protection rules, which defeats the purpose. The script's existence makes the apply reproducible; the admin's manual invocation is the audit point.
+
+## How to read the current GitHub state
+
+```bash
+gh api repos/ModernRelay/omnigraph/branches/main/protection
+```
+
+Outputs the live policy. Compare against `.github/branch-protection.json` to detect drift.
+
+## Why declared as code
+
+- **Audit trail**: `git log .github/branch-protection.json` shows every change with a reviewable diff and a merge commit.
+- **Disaster recovery**: if branch protection is accidentally removed or weakened via the UI, the JSON is the canonical recovery point.
+- **Consistency**: pairs with `.github/codeowners-roles.yml` (the CODEOWNERS source of truth). Repo policy lives in the repo.
+
+## What this gates
+
+After branch protection is applied, every PR targeting `main` must:
+
+1. Pass all listed status checks.
+2. Be up-to-date with `main` (rebase or merge-from-main).
+3. Have at least one approving review from a code owner for the touched paths.
+4. Have all review conversations resolved.
+5. Be squash- or rebase-merged (no merge commits).
+
+Even repo admins are subject to these rules.
+
+## Subsequent hardening (not in this PR)
+
+The branch-protection policy is the foundation. Future hardening adds:
+
+- **Required signed commits** (`required_signatures: true`) — once maintainers enroll GPG/SSH signing.
+- **Tag protection** for `v*` tags via `repos/.../tags/protection`.
+- **Required reviewers from specific teams** for high-leverage paths (e.g., `docs/invariants.md`) via CODEOWNERS tier expansion + the N-unique-approvers CI workaround.
+- **More required CI checks**: `cargo deny`, `cargo audit`, `cargo fmt --check`, `cargo clippy -D warnings`, CodeQL, secret scanning, schema-lint (MR-946).
+
+See the hardening playbook for the full plan.

scripts/apply-branch-protection.sh

@@ -0,0 +1,62 @@
+#!/usr/bin/env bash
+# Apply branch protection rules to the main branch.
+#
+# Requires:
+#   - `gh` CLI authenticated.
+#   - Repo-admin or org-admin permissions on ModernRelay/omnigraph.
+#
+# This script is idempotent: re-running applies whatever is currently
+# declared in .github/branch-protection.json. The JSON file is the
+# source of truth; this script is the apply mechanism.
+#
+# Usage:
+#   ./scripts/apply-branch-protection.sh                    # apply to main
+#   REPO=ModernRelay/omnigraph BRANCH=main ./scripts/...    # explicit
+#   DRY_RUN=1 ./scripts/apply-branch-protection.sh          # show what would apply
+
+set -euo pipefail
+
+REPO="${REPO:-ModernRelay/omnigraph}"
+BRANCH="${BRANCH:-main}"
+HERE="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+POLICY_FILE="${POLICY_FILE:-$HERE/../.github/branch-protection.json}"
+
+if [ ! -f "$POLICY_FILE" ]; then
+    echo "error: policy file not found: $POLICY_FILE" >&2
+    exit 1
+fi
+
+# Strip the _comment field — the GitHub API rejects unknown keys.
+PAYLOAD="$(mktemp)"
+trap 'rm -f "$PAYLOAD"' EXIT
+jq 'del(._comment)' "$POLICY_FILE" > "$PAYLOAD"
+
+if [ "${DRY_RUN:-0}" = "1" ]; then
+    echo "DRY RUN — would apply to $REPO/$BRANCH:"
+    echo
+    cat "$PAYLOAD"
+    exit 0
+fi
+
+echo "Applying branch protection to $REPO/$BRANCH from $POLICY_FILE"
+gh api -X PUT "repos/$REPO/branches/$BRANCH/protection" \
+    --input "$PAYLOAD" \
+    -H "Accept: application/vnd.github+json" \
+    > /dev/null
+echo "OK"
+
+# Verify by reading back.
+echo
+echo "Current policy on $REPO/$BRANCH:"
+gh api "repos/$REPO/branches/$BRANCH/protection" \
+    --jq '{
+        required_status_checks: .required_status_checks.contexts,
+        strict_status_checks: .required_status_checks.strict,
+        required_approvals: .required_pull_request_reviews.required_approving_review_count,
+        require_code_owner_reviews: .required_pull_request_reviews.require_code_owner_reviews,
+        enforce_admins: .enforce_admins.enabled,
+        linear_history: .required_linear_history.enabled,
+        force_pushes_allowed: .allow_force_pushes.enabled,
+        deletions_allowed: .allow_deletions.enabled,
+        conversation_resolution_required: .required_conversation_resolution.enabled
+    }'