Flatten harness branch names so a model's branch can't block them

Live run 3: the executor ran `git checkout -b <task id>` itself, and
publishing then failed, because git can't create '<task>/milestone-1'
while a branch named '<task>' exists. Milestone and delegation branches
are now '<task>-milestone-N' and 'delegation-milestone-N-F'. D4's
harness-owned branches (M2.1) remove the cause; this removes the
collision on the current loop.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPQ14L3LikyefCMFBqEbKE

nisse-dev/tests/test_quint_counterexamples.py

@@ -121,7 +121,7 @@ class Publisher:
         return {"open_pr": lambda intent: intent.payload["branch"] in self._gitea.prs}
 
     def publish(self, **kwargs: Any) -> PublishResult:
-        branch = f"{kwargs['task_id']}/milestone-{kwargs['milestone']}"
+        branch = f"{kwargs['task_id']}-milestone-{kwargs['milestone']}"
         self.attempts += 1
         intent = self._store.begin_intent("open_pr", {"branch": branch}, cause="t")
         if branch in self._gitea.prs:
@@ -135,7 +135,7 @@ class Publisher:
         )
 
     def published_pull_request(self, task_id: str, milestone: int) -> Any:
-        branch = f"{task_id}/milestone-{milestone}"
+        branch = f"{task_id}-milestone-{milestone}"
         if branch in self._gitea.prs:
             return PullRequest(number=20, state="open", head=branch, url="u")
         return None