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

docs/Architecture-Build-Spec.md

@@ -909,6 +909,9 @@ EXECUTING                       autonomous; no per-command approval
 PR_OPEN
   → open PR; react to review comments and commits (review loop)
   → on acceptance → next milestone, or project complete
+
+any waiting state (a sign-off gate, AWAIT_INPUT, PR_OPEN)
+  → [abandon]                  close the plan issue and any open PR, → IDLE
 ```
 
 ### B2. Requirements
@@ -917,8 +920,14 @@ PR_OPEN
   or the agent switches on its own under 0.2's rules; other projects stay parked at
   their last position and resume where they left off.
 - **Operator commands** (over transport): `use <project>`, `approve`,
-  `reject <feedback>`, `start milestone <n>`, `status`, `stop` (A4), plus free
-  text during planning (forwarded to the agent as answers).
+  `reject <feedback>`, `start milestone <n>`, `status`, `stop` (A4), `abandon`,
+  plus free text during planning (forwarded to the agent as answers).
+  - `abandon` ends the current work item from any waiting state: the "abandon"
+    among A4's continue / re-plan / abandon, and a close in 0.2's terms. It closes
+    the plan issue and any open milestone PR with a note, keeps the branches, and
+    marks any unresolved intent aborted so boot reconciliation does not gate on
+    work dropped on purpose. While work is running, `stop` comes first. Like
+    `status`, it is a command in every state, never read as an answer or feedback.
 - **Plan gating** per A4: architecture plan and each milestone plan require sign-off
   before the next phase. Enforced structurally. Plans and approvals are externalized
   to Gitea issue comments (Part H, decided); Matrix and Gitea are both approval