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.ts

@@ -23,6 +23,7 @@ import type { ConflictResolutionService } from '../coordination/conflict-resolut
 import { phaseBranchName, taskBranchName } from '../git/branch-naming.js';
 import { ensureProjectClone } from '../git/project-clones.js';
 import { createModuleLogger } from '../logger/index.js';
+import { phaseMetaCache, fileDiffCache } from '../review/diff-cache.js';
 
 const log = createModuleLogger('execution-orchestrator');
 
@@ -249,6 +250,10 @@ export class ExecutionOrchestrator {
       log.info({ taskId, taskBranch, phaseBranch, project: project.name }, 'task branch merged into phase branch');
     }
 
+    // Invalidate diff cache — phase branch HEAD has advanced after merges
+    phaseMetaCache.invalidateByPrefix(`${phaseId}:`);
+    fileDiffCache.invalidateByPrefix(`${phaseId}:`);
+
     // Emit task:merged event
     const mergedEvent: TaskMergedEvent = {
       type: 'task:merged',