Adds DiffCache<T> module, extends BranchManager with getHeadCommitHash, and wires phase-level caching into getPhaseReviewDiff and getFileDiff. Cache is invalidated in ExecutionOrchestrator after each task merges into the phase branch, ensuring stale diffs are never served after new commits. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>