Add a DELETE_BRANCH saga compensation kind and recovery handler

[hadamrd]

Parent: #431

Part of epic: "Compensate the branch a failed worker abandons"

Add CompensationKind.DELETE_BRANCH to tasks/saga.py and teach control/recovery.py to discharge it. Extend reconcile_stale_sagas() to accept a reap_branch(branch) callback and add DELETE_BRANCH to _HANDLED_COMPENSATION_KINDS so a saga carrying it is eligible for full compensation (not quarantine). Primary falsifiable acceptance: given a stale saga whose compensations include a DELETE_BRANCH targeting 'loop/42', reconcile invokes reap_branch('loop/42') exactly once and transitions the saga to COMPENSATED.

Customer story

As an operator who must not babysit orphan branches: when a stale worker's lease expires, recovery should delete the branch the saga created, not just its worktree.

[hadamrd]

forge-loop: worker iteration loop exhausted after max attempts.

• last state: dirty_no_commit
• worktree: /tmp/forge-forge-loop/wt-loop-432
• PR: (none)

The branch is left in place for human inspection. Run cd /tmp/forge-forge-loop/wt-loop-432 && git status to pick up where the loop stopped.

[hadamrd]

forge-loop attempt — ❌ FAILED (~810s)

Note: Exception: Command failed with exit code 143 (exit code: 143)
Error output: Check stderr output for details

fingerprint: d7b62ccaa31f

{
  "ts": "2026-06-08T19:07:33+00:00",
  "status": "failed",
  "pr_url": null,
  "duration_s": 810.1,
  "note": "Exception: Command failed with exit code 143 (exit code: 143)\nError output: Check stderr output for details",
  "event_count": 0,
  "brief_fingerprint": "d7b62ccaa31fd7369fcec165bae1d4d815eb3399bb75f31c0a904a053d843ef1"
}