fix: Show completed phase diffs in review tab

Completed phases showed "No phases pending review" because:
1. Frontend filtered only pending_review phases
2. Server rejected non-pending_review phases
3. After merge, three-dot diff returned empty (merge base moved)

Fix: store pre-merge merge base hash on phase, use it to reconstruct
diffs for completed phases. Frontend now shows both pending_review and
completed phases with read-only mode (Merged badge) for completed ones.

apps/server/execution/orchestrator.ts

@@ -278,6 +278,18 @@ export class ExecutionOrchestrator {
 
     const projects = await this.projectRepository.findProjectsByInitiativeId(phase.initiativeId);
 
+    // Store merge base before merging so we can reconstruct diffs for completed phases
+    for (const project of projects) {
+      const clonePath = await ensureProjectClone(project, this.workspaceRoot);
+      try {
+        const mergeBase = await this.branchManager.getMergeBase(clonePath, initBranch, phBranch);
+        await this.phaseRepository.update(phaseId, { mergeBase });
+        break; // Only need one merge base (first project)
+      } catch {
+        // Phase branch may not exist in this project clone
+      }
+    }
+
     for (const project of projects) {
       const clonePath = await ensureProjectClone(project, this.workspaceRoot);
       const result = await this.branchManager.mergeBranch(clonePath, phBranch, initBranch);