feat(review): validate CodeRabbit-style review options

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Summary

Audit, document, and test the CodeRabbit-style review options that have been implemented or planned for Gittensory reviews.

Context

Several review behavior options were modeled after CodeRabbit-style controls, but they need verification before they become a stable self-host configuration surface. The goal is to know which options exist, which are wired, and how each changes review behavior.

Requirements

• Inventory all CodeRabbit-style options currently implemented, partially implemented, or referenced in docs/config.
• Document defaults, allowed values, provider compatibility, and expected output changes.
• Add missing validation for unsupported or contradictory option combinations.
• Test options against representative PR fixtures.
• Keep review quality and gate safety above configurability.

Deliverables

• Option inventory with current status: working, partial, missing, or deprecated.
• Config docs and template entries for supported options.
• Focused tests for each supported option.
• Cleanup issues for options that should be removed or redesigned.

Acceptance criteria

• Each supported option has a test showing its review behavior impact.
• Unsupported options fail clearly or are omitted from templates.
• The default configuration remains conservative and suitable for self-hosted reviews.

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2026-07-01 issue audit: Duplicate of #1681 (same review-options inventory/validation pass). Canonical config-as-code tracking is #1959.