build: git-hooks - Add changelog validation #397

Build Summary

Run deterministic changelog validation locally before commits. Consume the existing changelog workstream; reviewed validator is partly a stub, so warnings-only validation must not be presented as enforcement.

Acceptance Criteria

• Agreed validator consumed, not duplicated.
• Valid examples pass; malformed required structure fails non-zero.
• Trigger behaviour documented.
• No generation, release creation or version bumps.

Definition of Ready

• Scope, fixtures, prerequisites and overlap identified.

Definition of Done

• Criteria pass; commands, revision, environment and exit codes recorded.
• Documentation reflects tested behaviour; applicable review/checks pass.

Branch instructions

Recheck repository instructions, active work and PR template before implementation; independently reviewable branch from develop, no automatic merged-branch reuse. Preserve unrelated changes; no force-push, merge or deployment. Estimate unset; no dates/milestone.