chore: clean up stale merged branches, add auto-cleanup workflow

[Shashank-Tripathi-07]

Problem

Merged branches never get cleaned up, so the repo has accumulated a pile of dead branches with no way forward.

Fix

Adds .github/workflows/infra-cleanup-stale-branches.yml: weekly, deletes any branch that's fully merged into dev and inactive 14+ days. Skips main, dev, gh-pages. Supports workflow_dispatch with a dry_run toggle for manual testing.

Only ever deletes branches whose commits are already an ancestor of dev, so it can't lose work.

Also included

30 branches already merged and safe to delete right now:

chore/precommit-cleanup, chore/staffml-ci-path, cleanup/book-validate-paths,
feat/container-preflight-urls, fix/badge-fixes, fix/callout-flow,
fix/staffml-reusable-concurrency, fix/staffml-trigger-on-workflow-edits, fmt-fix,
kai/clarify-community-map-totals, vol1/ch1-pass4, vol1/ch10-pass3, vol1/ch11-pass3,
vol1/ch12-pass3, vol1/ch13-pass3, vol1/ch14-pass3, vol1/ch2-final, vol1/ch2-pass3,
vol1/ch3-final, vol1/ch4-final, vol1/ch5-final, vol1/ch5-pass2, vol1/ch5-pass4,
vol1/ch6-pass4, vol1/ch7-pass3, vol1/ch8-final, vol1/ch8-pass3, vol1/ch9-final,
vol1/ch9-pass3, vol1/frontmater-final, vol1/frontmatter

@profvjreddi could you delete these (or grant access) when you get a chance?

Test plan

• workflow_dispatch with dry_run: true, confirm candidate list matches above
• Run for real, confirm branches removed
• Confirm main/dev/gh-pages and open-PR branches untouched

[Shashank-Tripathi-07]

I read this and went through this. Even though the idea I proposed earlier is quite right. We need to clean branches that are not deleted after their code is merged. But this hinders a lot of future workflows that we'll be building. I want to keep the future workflow launches smooth and don't want them to get into any trouble. I would insist in all maintainers having the attitude of deleting branches once their code is merged.

[Shashank-Tripathi-07]

Closed and branch deleted as required. :)