feat(devtools): prune merged local branches after a pull

[davidmoralesz]

What

Adds a shared post-merge job to lefthook/org.yml. After a git pull on the default branch it deletes local branches whose pull request has merged.

A branch is deleted only when all of these hold:

• its remote branch is gone
• it is not checked out in any worktree
• its work is in the default branch: the tip is an ancestor of it, or (for a squash merge, which leaves no ancestry) gh reports a merged PR whose head is exactly the local tip

Anything else is kept and named, so commits made after the merge, or a PR closed unmerged, are never lost. Without gh, a squash-merged branch is kept rather than guessed at. The script always exits 0, and --dry-run shows what it would do.

Files

• .devtools/prune-merged-branches.sh: the logic
• .lefthook/post-merge/prune-branches.sh: wrapper lefthook finds, like the pre-push ones
• lefthook/org.yml: the post-merge job
• .devtools/test-hooks.sh: cases for ancestry merge, gh-confirmed squash, squash with a moved head, closed unmerged, live remote, worktree, non-default branch, dry run
• README.md: short section

Testing

• bash .devtools/test-hooks.sh: passes, with gh stubbed
• shellcheck, prettier, gitleaks and commitlint via the repo's own hooks: pass
• End to end: a throwaway consumer pulling lefthook/org.yml through remotes pinned to a tag, then a real git pull after a merge. The ancestry-merged branch was deleted; the squash-merged (no PR for gh to confirm), closed and live branches were kept
• Not tested against real GitHub: the gh pr list path is covered only by the stub

Notes

• Consumers get this once a release is cut and their lefthook.yml bumps ref and runs lefthook install. Repos still on pre-commit need the lefthook migration first.
• While testing, ref: <branch> with refetch: true (the tip in AGENTS.md) made hooks report "script does not exist" when git ran them; a pinned tag without refetch worked. Cause not confirmed, left unchanged here.