nisse-review: a new head's review waits for its CI

A review judges the CI result (R1), so a new head's review now waits while
its checks are pending, and starts once they finish, passed or failed. It
also keeps reviews off a GPU that CI is using: #154's first review ran
beside CI's end-to-end job and took about 30 minutes.

- Pending: waits up to tuning.review.ci_wait_s (1 h), then reviews anyway.
- No checks: waits tuning.review.ci_grace_s (2 min) for them to appear,
  then reviews.
- A PR waiting on CI holds up no other: the oldest settled one goes first.
- The head's first sighting is kept on the item (seen_at).

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01J5Utyfzw7gF7UhjRDEtjTk

nisse-review/src/nisse_review/state.py

@@ -33,6 +33,7 @@ class PR:
     head: str = ""  # the commit last seen at the PR's head
     reviewed: str = ""  # the commit its review was posted on
     attempts: int = 0  # failed runs at this head
+    seen_at: float = 0.0  # when this head was first seen (the CI wait)
 
 
 def item_id(repo: str, number: int) -> str: