[bug-005] Disable per-marker verifier by default; gate behind OLAVA_VERIFIER env

Verifier (feat-007a) was awaiting all in-flight per-marker Olava calls
at end-of-turn before sending [DONE]. Per-call latency observed 12-17s
on olava-extract, and 3-of-4 calls came back empty in practice. This
added ~12s to time-to-[DONE] on every chat with citations.

Empirically the model emits a clean <CITATIONS> JSON block on its own
the vast majority of the time - citations land via the existing block-
parser path regardless. The verifier is mostly redundant work today.

Single env-gated flag: when OLAVA_VERIFIER is unset (or anything other
than "on"), the pre-extract is skipped, verifierDocId stays null, and
the existing fireVerifier early-return turns marker detection into a
no-op. verifierPromises stays empty so the end-of-turn await collapses.

All supporting code preserved (verifyCitation, marker detection,
streaming SSE emit) so re-enabling is just OLAVA_VERIFIER=on if/when
we observe the model regressing on the JSON tail.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>