Wire delegation to A12's trigger, and implement A3's resume rule

Two gaps found by auditing spec against code, the second created by
7b landing the harness without its boot-time half.

**The harness was not connected to anything.** Only a live test
constructed a Delegation; A12's failure-count trigger still gated to
the human, which A13 describes as the fallback rather than the path.
Consecutive command failures now hand the sub-task to the delegate,
with the failures in the brief so it need not rediscover what the
local model learned the hard way. A usable handback lets the local
loop carry on; anything else escalates, because a cap breach leaves a
half-edited tree.

**A3 step 5 was unimplemented.** A crash mid-delegation failed safe —
boot gated on "restarted mid-EXECUTING" — but always gated, even on
the clean cut the spec says should resume, and left the branch orphaned
because _abandon only runs while the process lives.

- A delegation now records an intent before the branch exists, and
  settles it whichever way the delegation ends; an aborted delegation
  is a finished one, and leaving it pending would make the next boot
  believe work was still in flight
- delegation_verdict implements the clean-cut test as an observable
  check, never model self-assessment: settled if a completion follows
  the start, absent if the branch was never created, gate on a
  tool_call with no recorded result, resume otherwise
- recover() sweeps at boot, announcing a resume with a notice rather
  than a question, since a clean cut needs no decision from the
  operator, and leaving an unclean one pending so the generic sweep
  gates on it
- resume() continues the same delegation rather than starting a
  second: no new branch, no second delegation_started, because A10
  defines a session as the delegation, not the process lifetime

Honest limit, stated in the code: "the branch sits at the expected
commit" is approximated by the branch existing and carrying the work,
since per-turn shas are not recorded in the log, so a branch rewritten
underneath us would go undetected.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

nisse-dev/src/nisse_dev/git.py

@@ -66,6 +66,16 @@ class GitRepo:
     def checkout_new_branch(self, name: str) -> None:
         self._run("checkout", "-q", "-b", name)
 
+    def checkout_existing_branch(self, name: str) -> None:
+        self._run("checkout", "-q", name)
+
+    def branch_exists(self, name: str) -> bool:
+        try:
+            self._run("rev-parse", "--verify", "--quiet", f"refs/heads/{name}")
+        except GitError:
+            return False
+        return True
+
     def commit_all(self, message: str) -> str | None:
         """Returns the new commit sha, or None when there was nothing to
         commit — an empty milestone is a fact to report, not an error."""