feat(maintenance): auto-sweep stale branches after merge + periodic sweep

[hadamrd]

Problem

After every merge the loop accumulates branches indefinitely:

• This session: 115+ branches on hadamrd/forge-loop, 46 still on remote after merges.
• Loop already uses gh pr merge --squash --auto --delete-branch on its auto-merge path, but NOT all merge paths go through that — and the local branch in the operator's checkout never gets deleted.
• Operator must manually run git push origin --delete <br> loops + git branch -D loops. Painful + error-prone (one slip and a non-merged branch dies).

Acceptance

1. Per-merge cleanup (already partial): every successful auto-merge MUST delete the remote branch. Add the same to admin-merge and rescue-merge paths.
2. Per-tick sweep: every Nth tick (default 20), call the new sweep:
   • List remote branches matching loop/*, feat/*, fix/*, refactor/*, chore/*, test/*, docs/*.
   • For each: query the GitHub API for the PR. If state is MERGED or CLOSED (and > 7 days old, configurable), delete the remote branch via gh api -X DELETE.
   • Local branch cleanup: any branch with no remote tracking ref + last commit older than 30 days → delete.
3. Emit typed event branch_sweep_done with counts (deleted/skipped/error).
4. forge-loop sweep branches CLI command for manual runs.

Test plan

• Unit: mock GhClient with mixed MERGED/CLOSED/OPEN PRs; assert correct delete calls.
• Adversarial: a branch with no PR at all (orphan) — don't crash, log + skip.
• Adversarial: gh API rate limit during sweep — back off + resume, don't hammer.
• Regression: a branch that has an OPEN PR is NEVER deleted.

File pointers

• src/forge_loop/branch_sweep.py (new)
• src/forge_loop/runner/tick.py — call sweep on the maintenance_every_n_ticks cadence
• src/forge_loop/cli.py — sweep branches subcommand
• src/forge_loop/settings.py — maintenance.branch_sweep_every_n_ticks (default 20), maintenance.branch_min_age_days (default 7)
• tests/test_branch_sweep.py (new)

Customer story

A platform team runs forge-loop on their repo for a week. Without this, they have 200+ stale branches in their list. With this, the branch list stays clean — only OPEN PRs and the default branch.