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>

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.