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/trpc/routers/phase.ts

@@ -11,6 +11,7 @@ import { requirePhaseRepository, requireTaskRepository, requireBranchManager, re
 import { phaseBranchName } from '../../git/branch-naming.js';
 import { ensureProjectClone } from '../../git/project-clones.js';
 import type { FileStatEntry } from '../../git/types.js';
+import { phaseMetaCache, fileDiffCache } from '../../review/diff-cache.js';
 
 export function phaseProcedures(publicProcedure: ProcedureBuilder) {
   return {
@@ -237,6 +238,16 @@ export function phaseProcedures(publicProcedure: ProcedureBuilder) {
         const projects = await projectRepo.findProjectsByInitiativeId(phase.initiativeId);
         const files: FileStatEntry[] = [];
 
+        if (projects.length === 0) {
+          return { phaseName: phase.name, sourceBranch: phBranch, targetBranch: initBranch, files: [], totalAdditions: 0, totalDeletions: 0 };
+        }
+
+        const firstClone = await ensureProjectClone(projects[0], ctx.workspaceRoot!);
+        const headHash = await branchManager.getHeadCommitHash(firstClone, phBranch);
+        const cacheKey = `${input.phaseId}:${headHash}`;
+        const cached = phaseMetaCache.get(cacheKey);
+        if (cached) return cached;
+
         for (const project of projects) {
           const clonePath = await ensureProjectClone(project, ctx.workspaceRoot!);
           const entries = await branchManager.diffBranchesStat(clonePath, diffBase, phBranch);
@@ -255,7 +266,7 @@ export function phaseProcedures(publicProcedure: ProcedureBuilder) {
         const totalAdditions = files.reduce((sum, f) => sum + f.additions, 0);
         const totalDeletions = files.reduce((sum, f) => sum + f.deletions, 0);
 
-        return {
+        const result = {
           phaseName: phase.name,
           sourceBranch: phBranch,
           targetBranch: initBranch,
@@ -263,6 +274,8 @@ export function phaseProcedures(publicProcedure: ProcedureBuilder) {
           totalAdditions,
           totalDeletions,
         };
+        phaseMetaCache.set(cacheKey, result);
+        return result;
       }),
 
     getFileDiff: publicProcedure
@@ -297,6 +310,13 @@ export function phaseProcedures(publicProcedure: ProcedureBuilder) {
         const decodedPath = decodeURIComponent(input.filePath);
 
         const projects = await projectRepo.findProjectsByInitiativeId(phase.initiativeId);
+
+        const firstClone = await ensureProjectClone(projects[0], ctx.workspaceRoot!);
+        const headHash = await branchManager.getHeadCommitHash(firstClone, phBranch);
+        const cacheKey = `${input.phaseId}:${headHash}:${input.filePath}`;
+        const cached = fileDiffCache.get(cacheKey);
+        if (cached) return cached;
+
         let clonePath: string;
         if (input.projectId) {
           const project = projects.find((p) => p.id === input.projectId);
@@ -305,18 +325,22 @@ export function phaseProcedures(publicProcedure: ProcedureBuilder) {
           }
           clonePath = await ensureProjectClone(project, ctx.workspaceRoot!);
         } else {
-          clonePath = await ensureProjectClone(projects[0], ctx.workspaceRoot!);
+          clonePath = firstClone;
         }
 
         const git = simpleGit(clonePath);
         // Binary files appear as "-\t-\t<path>" in --numstat output
         const numstatOut = await git.raw(['diff', '--numstat', `${diffBase}...${phBranch}`, '--', decodedPath]);
         if (numstatOut.trim() && numstatOut.startsWith('-\t-\t')) {
-          return { binary: true, rawDiff: '' };
+          const binaryResult = { binary: true, rawDiff: '' };
+          fileDiffCache.set(cacheKey, binaryResult);
+          return binaryResult;
         }
 
         const rawDiff = await branchManager.diffFileSingle(clonePath, diffBase, phBranch, decodedPath);
-        return { binary: false, rawDiff };
+        const result = { binary: false, rawDiff };
+        fileDiffCache.set(cacheKey, result);
+        return result;
       }),
 
     approvePhaseReview: publicProcedure