feat: add in-memory diff cache with TTL and commit-hash invalidation

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>

apps/server/execution/orchestrator.test.ts

@@ -48,6 +48,7 @@ function createMocks() {
     diffBranches: vi.fn().mockResolvedValue(''),
     diffBranchesStat: vi.fn().mockResolvedValue([]),
     diffFileSingle: vi.fn().mockResolvedValue(''),
+    getHeadCommitHash: vi.fn().mockResolvedValue('deadbeef00000000000000000000000000000000'),
     deleteBranch: vi.fn(),
     branchExists: vi.fn().mockResolvedValue(true),
     remoteBranchExists: vi.fn().mockResolvedValue(false),