Creates diff-cache.ts module with generic DiffCache<T> class (TTL, prefix invalidation, env-var configuration) and exports phaseMetaCache / fileDiffCache singletons. Wires cache into getPhaseReviewDiff via getHeadCommitHash on BranchManager. Adds 6 unit tests for DiffCache and 5 integration tests verifying cache hit/miss behaviour, prefix invalidation, and NOT_FOUND guard. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>