Document how to clean up merged feature branches

[sebastientaggart]

Adds a short "Cleaning up merged branches" section to the end of docs/branching.md.

Frames branch cleanup as optional housekeeping (Code Cannon has no opinion), recommends GitHub's auto-delete-on-merge setting as the zero-effort baseline, and provides copy-pasteable one-liners for manual local and remote cleanup using --merged as the safety filter. Includes a warning against age/staleness-based pruning.

Docs-only change. No skill or config changes.

Issue #60

[sebastientaggart]

Review Summary

Verdict: APPROVE

Findings

• [NOTE] The local-branch cleanup one-liner uses grep -vE '^\*|^\s*(main|dev|test)$', which only excludes those names when the integration branch is literally dev; users who set BRANCH_DEV to something else (e.g. develop) will need to adjust both the git branch --merged argument and the grep exclusion list, which the surrounding prose doesn't quite spell out.