CLAUDE.md: post-merge feature-branch cleanup rule

[avanelsas]

Summary

Adds a short rule to CLAUDE.md documenting how to clean up
after a PR is squash-merged — and crucially, the order:

1. Verify the squash commit is on `origin/main`.
2. Delete the feature branch locally (`git branch -D` —
   `-d` refuses because the squash creates a new commit
   that doesn't share history with the feature branch).
3. Delete the feature branch on the remote.

The "main has been pulled / squash verified" precondition is
deliberate — pre-emptive branch deletion risks losing
in-flight work if the merge was never completed or got
reverted.

Slots in immediately after the existing PR readiness gate
section so the lifecycle reads top-to-bottom: gates → ask
before opening → cleanup after merge.

Test plan

• `clj-kondo --lint src test scripts` — 0 errors / 0 warnings.
• `cljfmt check src test scripts` — all files formatted.
• `npx shadow-cljs compile test` — 486 tests / 1436
  assertions / 0 failures / 0 errors.
• `npx shadow-cljs release app` — 0 build warnings.
• Doc-only change; no runtime behaviour to verify.

🤖 Generated with Claude Code