nisse-review: later reviews, replies, rounds, stop and review (M7.5)

Back on a PR it reviewed: a new head gets a later review of the commits
since (each one's diff, through the API) and of its earlier points; what
was said since gets replies, answered where it was said or skipped with a
reason. It signs off once and stays quiet while still happy. Past
tuning.review.max_rounds it sums up for the operator and goes quiet,
until a push or someone other than the PR's author joins in. An
operator's `stop` quiets it; `review` asks for a fresh review.

Reviews carry numbered markers and replies their point's key, so
recovery posts nothing twice.

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/config.py

@@ -59,6 +59,8 @@ class ReviewTuning:
     # are pending, and ci_grace_s for checks to appear at all. 0: don't wait.
     ci_wait_s: float = field(default=3600.0, metadata={"zero": True})
     ci_grace_s: float = field(default=120.0, metadata={"zero": True})
+    # Reviews and reply sets before it sums up for the operator (R2).
+    max_rounds: int = 4
 
 
 @dataclass(frozen=True)