review-classifier: ask a check once more when its answer isn't valid JSON

An answer that fails to validate is asked for a second time, for that
classification only, with a reminder to reply with the JSON object alone.
The second ask comes out of the classification's CLASSIFICATION_DEADLINE:
it gets what the first ask left, and is skipped when less than 20 seconds
remain, so RUN_DEADLINE's bound still holds. A failed ask (runtime error,
timeout) is not repeated. If the second answer does not validate either,
the row has no answer, as before.

Model::ask takes the deadline for the ask; the ACP runtime no longer holds
its own.

The parser also takes the first fenced block of a reply that has prose
around the fence; prose around a bare object is still rejected.

swarm-review-classifier/README.md

@@ -22,6 +22,8 @@ forge and holds no forge credential.
 - An answer is a JSON object with a verdict, a certainty and a reason; the
   prompt asks for a reason under 350 characters, and the classifier truncates one over the 500 cap to 499 plus `…`. A failed run, a
   timeout or an answer that doesn't validate leaves that row without a verdict, and it doesn't count.
+  An answer that doesn't validate is asked for once more first, with a
+  reminder to reply with the JSON object alone, within the same three minutes.
 - A run with no API key, or with no classification answered, fails as a
   whole: the caller sees an error, never a verdict.