Document changelog fragment validation tooling and add precommit check

[Widthdom]

Summary

.codex/workflows/changelog-fragment.md lines 26-27 and changelog.d/README.md lines 43-44 both state that non-issue fragments must "omit the issues field entirely" and explicitly warn against writing issues: null or issues: []. However, there is no validation or tooling documented to enforce this constraint. Agents following the fragment template may accidentally write issues: null believing the validation pipeline will catch it, but the documentation does not reference the validation command or tool that would catch such errors.

Where

• .codex/workflows/changelog-fragment.md:26-27
• changelog.d/README.md:43-44
• .codex/workflows/release-changelog.md:25-26 (references validation but no detail)
• tools/CodeIndex.Changelog/ (no validation detail documented)

Suggested approach

1. Document the changelog fragment validation command explicitly in .codex/workflows/release-changelog.md
2. Add a pre-commit check in .codex/workflows/precommit.md that validates changelog fragments before commit
3. Include example error messages that agents would see if they write issues: null
4. Add a "Validation" section to changelog-fragment.md explaining how to test fragments locally
5. Reference the validation tool in both the fragment template and README
6. Consider adding a CI check that blocks PRs with malformed fragments (if not already present)

[Widthdom]

Work started on branch codex/fix-issue1999-1849.