Add changelog/unreleased-entry-required review rule

[bborbe]

What

New review rule changelog/unreleased-entry-required (SHOULD, judgment, @commits trigger) added to docs/changelog-guide.md; rules/index.json regenerated via make build-index.

Flags any PR that changes non-vendored source in a repo that has a CHANGELOG.md but adds no ## Unreleased bullet. SHOULD → Should Fix → request-changes, so the maintainer bot won't auto-merge a PR that would leave the release agent nothing to promote.

Why

In an autoRelease: true repo, the release agent renames ## Unreleased → ## vX.Y.Z post-merge. A PR merged with an empty ## Unreleased makes the release agent no-op — the change lands on master but never ships to consumers. This rule is the forcing function that keeps every merge releasable.

Coverage

One insertion point (rule index) feeds all three consumers: /coding:pr-review, /coding:code-review, and the maintainer bot (its execution phase inlines pr-review.md at runtime). No per-repo or bot-side change.

Scope / detection

• Gate on CHANGELOG.md presence at repo root (not autoRelease) — manual-release repos also need the section current.
• Judgment tier waives trivial diffs (comment/whitespace-only, or edits to the changelog guide itself).
• Dogfooded: this PR adds its own ## Unreleased entry.

make precommit green (check-links, check-json, check-index, check-coverage 30/30, check-acceptance).

[ben-s-pull-request-reviewer]

No worktree to remove — working directly in the checkout.

{
  "verdict": "approve",
  "summary": "PR adds a well-structured judgment rule `changelog/unreleased-entry-required` with clear `applies_when`, `enforcement`, `Why`, and examples. The PR dogfoods correctly by including its own `## Unreleased` bullet with `feat:` prefix. `rules/index.json` entry is valid JSON, all fields present. No issues found.",
  "comments": [],
  "concerns_addressed": [
    "correctness: rule logic in docs/changelog-guide.md correctly gates on CHANGELOG.md presence and checks for a +bullet under ## Unreleased; waiver for comment/whitespace/guide-only diffs is well-reasoned",
    "correctness: rules/index.json new entry is valid JSON with all required fields (id, anchor, applies_when, enforcement, enforcement_type, level, owner, trigger, doc_path)",
    "correctness: CHANGELOG.md PR dogfoods correctly — ## Unreleased bullet added with feat: prefix, placed after preamble before ## v0.31.0"
  ]
}