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>

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/) |