nisse-review: read long files in parts; a retry starts afresh

From its failed review of #156: read_file stopped at 20,000 characters
with no way on, so the reviewer hunted each file's end a definition at a
time and ran out of turns; its retry carried every read before it, and a
model call timed out. Both drafts were lost.

- read_file takes a start line and says where the next part starts.
- Each run's view starts at its brief (Transcript.since); the log keeps
  the rest.
- A run out of turns with points drafted posts them as a partial review.

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/review.py

@@ -118,6 +118,10 @@ class Reviewer:
         """The review run: it ends waiting on post_review, or not."""
         transcript = self._store.transcript(item_id, "driver")
         start = transcript.read()[-1].id if transcript.read() else 0
+        # Each run starts afresh from its brief: a retry doesn't carry the
+        # failed run's reads (#156: they filled the context, and a call timed
+        # out). The log keeps them.
+        transcript = transcript.since(start)
         self._brief(transcript, repo.repo, pr)
         project = _project(repo.repo)
         index = self._knowledge.index(item_id, project)