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/src/nisse_dev/executor.py

@@ -134,7 +134,7 @@ class LLMExecutor:
         result = self._delegate.run(
             transcript.as_actor("helper"),
             brief=brief,
-            branch=f"delegation/milestone-{milestone}-{failures}",
+            branch=f"delegation-milestone-{milestone}-{failures}",
         )
         if result.usable:
             return None