Add the abandon command

A4 already names abandon as one of the ways out of a halt (continue,
re-plan, or abandon) and 0.2 closes a work item as completed or
abandoned, but no command did it: from a gate or a question the only way
to drop a task was to edit the state database.

- 'abandon' ends the task from any waiting state and returns to IDLE.
  The move is made durable first; then the plan issue and any open
  milestone PR are closed with a note (branches kept), and any
  unresolved intent is aborted so boot reconciliation does not gate on
  work dropped on purpose. A Gitea failure is reported, not fatal.
- Like 'status' it is a command in every state, never read as an answer
  or feedback. While work runs, 'stop' comes first, as before.
- The restart / stranded question and the PR_OPEN hints mention it.
- Spec: B1 gains the abandon edge and B2 the command.

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/publish.py

@@ -102,6 +102,12 @@ class MilestonePublisher:
         except GiteaError:
             pass
 
+    def close_pull_request(self, index: int, note: str) -> None:
+        """Close a milestone PR with a note saying why. The branch is kept,
+        so the work can be reopened in Gitea."""
+        self._client.post_comment(self._repo, index, note)
+        self._client.close_pull_request(self._repo, index)
+
     def push_followup(
         self, *, task_id: str, milestone: int, message: str
     ) -> str | None: