feat: make GitHub reviews incremental and idempotent

[EmersonBraun]

Summary

Closes #46.

• Adds bounded, timeout-controlled GETs with one idempotent retry and no automatic POST/PATCH retry.
• Adds SHA/policy fingerprints and marker reconciliation to skip duplicate provider work and update one summary comment.
• Uses GitHub compare scope only when the previous marked SHA is an ancestor; missing/changed policy or force-push falls back to the full PR file list.
• Detects fork PRs on the safe pull_request boundary and exits SKIPPED/2 before provider execution.
• Adds offline tests for markers, retry, fork detection, and incremental scope; updates operations/changelog docs.

Verification

• npm run check
• npm pack --dry-run
• 53 tests passing locally.