Add PR comment intake and the review loop (B2) — step 6b

- poller.py: CommentPoller polls PR comments rather than taking
  webhooks, keeping the VM outbound-only (B3). On an outage it backs
  off exponentially, caps, and retries indefinitely while logging:
  polling is pure intake, so an outage delays a reaction but never
  fails a task or forces a transition
- prime() marks the existing thread as seen without delivering it, so
  starting a watch does not re-address feedback already handled
- Review loop: a comment in PR_OPEN enters the canonical transcript,
  the executor responds, and the fix is pushed to the same branch with
  a reply on the PR. No intent is recorded for a follow-up push — the
  PR already exists, so a repeated push is harmless, unlike opening a
  second PR
- Comment authors are checked against a separate Gitea operator
  allowlist: a PR comment is an instruction channel into an agent that
  writes code, so it is operator-scoped, and a Gitea login is not a
  Matrix MXID (A4 per-channel allowlists)
- Orchestrator now holds a lock: transport and the poller deliver on
  separate threads, and state transitions must not interleave
- Backoff timings are injectable, so the outage path is tested in
  milliseconds instead of being untestable

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

nisse-dev/src/nisse_dev/publish.py

@@ -10,7 +10,12 @@ an idempotent transition reconciled against external truth.
 from dataclasses import dataclass
 
 from nisse_dev.git import GitRepo, authenticated_url
-from nisse_dev.gitea import GiteaClient, PullRequest, pull_request_reconciler
+from nisse_dev.gitea import (
+    GiteaClient,
+    GiteaError,
+    PullRequest,
+    pull_request_reconciler,
+)
 from nisse_dev.reconcile import Reconciler
 from nisse_dev.state import StateStore
 
@@ -86,6 +91,32 @@ class MilestonePublisher:
         self._store.commit_intent(intent_id, cause=f"opened PR #{pr.number}")
         return PublishResult(pr, f"opened PR #{pr.number}: {pr.url}")
 
+    def comment(self, index: int, body: str) -> None:
+        """Reply on the PR. Courtesy only — the pushed commit is the real
+        answer — so a failure here must not derail the review loop."""
+        try:
+            self._client.post_comment(self._repo, index, body)
+        except GiteaError:
+            pass
+
+    def push_followup(
+        self, *, task_id: str, milestone: int, message: str
+    ) -> str | None:
+        """Push a fix commit onto an open milestone's branch.
+
+        No intent is recorded: the PR already exists, so a crash mid-push
+        costs a repeated push of the same commit, which is harmless. That is
+        the difference from publish(), where the side effect is a new PR.
+        """
+        branch = self.branch_for(task_id, milestone)
+        if self._git.current_branch() != branch:
+            self._git.checkout_new_branch(branch)
+        sha = self._git.commit_all(message)
+        if sha is None:
+            return None
+        self._git.push(self._remote, branch, token=self._token)
+        return sha
+
 
 def _pr_body(milestone: int, plan: str, service_url: str | None) -> str:
     parts = [