fix: fail closed on unparsed review output

[unohee]

Summary

• normalize a prefixed OpenSwarm JSON review document before reading action outputs
• fail fail-on-gate-not-run whenever no parsed gateRan: true verdict exists, including exit code 0

Evidence

• External cgf-portal E2E run 35479878890 exposed a real false-green: a late memory diagnostic prefixed the JSON document, the action emitted no changes, and the job passed despite gate-ran=false.
• Reproduced the exact prefixed JSON shape locally; the new extractor recovered approve and gateRan=true.
• git diff --check.

[unohee]

🔍 Fresh review of 2911fe7 (openswarm pr review --fresh)

✓ Decision: APPROVE
Verified action.yml JSON parsing robustness (diagnostic noise, gate-not-run detection, clean-file extraction), intentional LanceDB downgrade for cross-platform npm ci, and CHANGELOG consistency. No defects found in the current diff.
Suggestions:
- Consider adding a similar parse-validation guard for the SARIF output path if the CLI can also embed diagnostics in SARIF stdout.