Align PR review summaries with deterministic state

[gontzess]

Why

The PR review workflow needs the same deterministic summary/state handling that landed in the related workflow repo. This keeps incremental reviews from trusting model-written legacy state and adds effective interdiff handling for base drift or diverged compares.

What this changes

• Posts review summaries from an owned finalizer instead of having the model create or edit them
• Requires run_id before a previous marker is trusted for incremental or interdiff mode
• Adds same-repo effective interdiff and unchanged modes, with full review fallback
• Keeps separate connector/general summary headings
• Adds focused tests for state parsing, interdiff handling, and finalizer behavior

Validation

• Local Python tests, Python compile checks, YAML parsing, and git diff --check passed
• Cross-review completed with no new blocking findings
• Private canary PR testing passed for connector full, incremental, unchanged, and interdiff modes
• Private canary PR testing passed for the general review profile and summary heading

Draft while final review is pending.