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>

.env.example

@@ -31,3 +31,6 @@ NISSE_GITEA_URL=https://gitea.example
 NISSE_GITEA_TOKEN=your-scoped-service-account-token
 # Sandbox repo for live tests; never the real project repo.
 NISSE_GITEA_TEST_REPO=owner/sandbox-repo
+# Gitea logins allowed to drive the agent via PR comments (A4 per-channel
+# allowlist; a Matrix MXID and a Gitea login are different names).
+NISSE_GITEA_OPERATORS=your-gitea-login