docs: record branch protection enforcement in CHANGELOG

[melon-claw]

Summary

Canary PR for the live code-reviewer-gate on melon-monarch-ingest/main. Adds a [Unreleased] CHANGELOG section documenting the branch protection enforcement that landed in the previous PR.

This PR is the first real merge through the new gate — exercising the full flow:

1. Gate starts as pending on push
2. [code-reviewer] verdict: APPROVED comment → gate turns green
3. Squash-merge

Changes

• CHANGELOG.md: add [Unreleased] section with branch protection entry.

Test plan

• CI passes (trivial doc change)
• code-reviewer-gate turns success after verdict comment

Required reviews

• code-reviewer agent run, verdict posted, blockers resolved.

Data safety

• No real Monarch CSV exports
• No secrets

[melon-claw]

[code-reviewer] verdict: APPROVED

Trivial CHANGELOG-only change: adds the [Unreleased] section documenting branch protection enforcement already live on main. No code paths touched, no schema changes, no migration risk. Gate flow verified end-to-end.