fix(gemini): recover JSON review when model emits prose before the envelope

[AlgizPure]

Problem

parseReviewOutput (plugins/gemini/scripts/lib/gemini.mjs) strips a markdown code-fence only when it sits at offset 0:

.replace(/^```(?:json)?\s*/i, "").replace(/\s*```$/i, "")

When Gemini streams narration before the JSON envelope — e.g.

I've checked .gitignore. Now let's check reindex_cases.py.
```json
{ "verdict": "...", "findings": [...] }

— the fence is no longer at offset 0, so `JSON.parse` runs on the prose and throws `REVIEW_PARSE_ERROR`. The entire review (a perfectly valid envelope sitting later in the output) is discarded and the lane reports a failure. This shows up often on research-heavy reviews where the model narrates its checks before converging to JSON.

## Repro

Feed `"some prose\n\`\`\`json\n{...valid review...}\n\`\`\`"` to `parseReviewOutput` → it throws instead of parsing the review that is right there.

## Fix

Additive `extractJsonCandidate` fallback in the `catch`: on parse failure, recover the JSON before giving up — take the **last** fenced ```` ```json ```` block (else the first-`{`..last-`}` run) and parse that; only throw `REVIEW_PARSE_ERROR` if that also fails.

- **Happy path unchanged** — clean output parses on the first `JSON.parse` and never reaches the fallback.
- **Genuinely non-JSON output still throws** `REVIEW_PARSE_ERROR`.

## Test

clean output : OK
fenced-only output : OK
prose + fenced (the bug): OK ← now recovered
prose + bare braces : OK
non-JSON garbage : throws REVIEW_PARSE_ERROR (unchanged)