perf: speed up slow git tests from ~18s to ~5.5s

- branch-manager: beforeEach→beforeAll (all 12 tests are read-only)
- worktree manager: clone template repo per test instead of full init
- signal-manager: reduce setTimeout delay from 100ms to 30ms

apps/server/agent/lifecycle/signal-manager.test.ts

@@ -133,7 +133,7 @@ describe('FileSystemSignalManager', () => {
       // Write signal after a delay
       setTimeout(async () => {
         await writeFile(signalPath, JSON.stringify(expectedSignal));
-      }, 100);
+      }, 30);
 
       const signal = await signalManager.waitForSignal(agentWorkdir, 1000);
       expect(signal).toEqual(expectedSignal);