Externalise plans to Gitea and accept approvals there (A4, Part H)

- approvals.py: PlanBoard opens one issue per project and posts each
  plan revision as a comment carrying its rev tag. The state store
  keeps only a pointer, so gate state can be re-derived from Gitea
  after a state-volume loss — external truth wins
- Gitea comments are a second approval source: revision binding, the
  typed word approve, and first-approval-wins work exactly as over
  transport, with a per-channel allowlist because a Gitea login is
  not a Matrix MXID
- Outcomes are echoed to the other channel, so the two records cannot
  silently disagree about what was approved
- A Gitea outage degrades durability of the record, not the ability
  to approve: planning proceeds and Matrix remains a complete channel
- Live test now asserts the server refuses the agent's own merge and
  its push to the protected branch. That property was previously
  true only because our code did not attempt it; branch protection
  makes it structural, and this test fails if the protection is ever
  relaxed
- Fixed a race in the process-group test: killing a group is
  asynchronous and the grandchild is an orphan, so its exit is not
  synchronised with our return; the test now waits for death

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/integration/test_gitea_live.py

@@ -92,8 +92,9 @@ def test_push_open_pr_and_reconcile(tmp_path: Path) -> None:
         # A4 is only real if the server enforces it. The agent's own token
         # must not be able to merge the PR it just opened, and must not be
         # able to push to the protected default branch at all.
-        merged = client._http.post(f"/repos/{repo_path}/pulls/{pr.number}/merge",
-                                   json={"Do": "merge"})
+        merged = client._http.post(
+            f"/repos/{repo_path}/pulls/{pr.number}/merge", json={"Do": "merge"}
+        )
         assert merged.status_code >= 400, (
             "the agent merged its own PR: branch protection is not enforcing "
             "the human gate"