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/approvals.py

@@ -74,6 +74,12 @@ class PlanBoard:
         except GiteaError:
             pass
 
+    def close(self, issue: int, note: str) -> None:
+        """Close the plan issue with a note saying why (e.g. abandoned).
+        Unlike echo, failures raise: the caller reports them."""
+        self._client.post_comment(self._repo, issue, note)
+        self._client.close_issue(self._repo, issue)
+
     def comments_since(self, issue: int, after_id: int) -> list[Comment]:
         return [
             c for c in self._client.list_comments(self._repo, issue) if c.id > after_id