docs(claude): add post-merge feature-branch deletion to Git Commit Policy

After today's WS-6 feat-branch audit revealed ten stale branches that
had been merged via squash/cherry-pick paths but never deleted, codify
the cleanup expectation directly in CLAUDE.md. Each feature branch is
expected to be deleted locally and on origin immediately after merge —
not "eventually" — to prevent SHA-illusion confusion in future audits.

Co-Authored-By: Claude <noreply@anthropic.com>

CLAUDE.md

@@ -359,6 +359,12 @@ docs: short description of documentation changes
 test: short description of test additions
 ```
 
+- After a feature branch is merged into main, delete it locally
+  (`git branch -d <branch>`) and remotely (`git push origin --delete
+  <branch>`) immediately. Stale feature branches accumulate cognitive
+  load and obscure SHA-level history when squash-merges leave commit
+  hashes orphaned on the original branch.
+
 Examples:
 - `feat: person tags system with org-level skills and sync endpoint`
 - `fix: auth race condition on page refresh`