docs: document changeset branch hygiene — prevent cross-PR changeset leaks #754

Problem

When a feature branch carries commits from other PRs (e.g., via merge or cherry-pick), their .changeset/*.md files leak into the new PR's diff. This happened on PR #753 which accidentally included #752's pr-readiness-checks.md changeset.

Root Cause

Branch scope pollution — the branch wasn't rebased cleanly from dev before the PR was opened.

Fix

Add guidance to CONTRIBUTING.md about changeset hygiene:

1. Always rebase from dev before opening a PR
2. Check \git diff origin/dev --name-only -- .changeset/\ — you should only see YOUR changeset file(s)
3. If you see someone else's changeset, revert it: \git checkout origin/dev -- .changeset/{file}\

Optional CI Enhancement

The PR readiness workflow could warn when changeset files in the diff don't match the PR's scope (future work, low priority).

Next Steps

Update CONTRIBUTING.md with a 'Changeset Hygiene' section covering the above guidance.