Phase 4: Atomic operations and stress testing (bd-114, bd-110)

Completes daemon architecture implementation:

Features:
- Batch/transaction API (OpBatch) for multi-step atomic operations
- Request timeout and cancellation support (30s default, configurable)
- Comprehensive stress tests (4-10 concurrent agents, 800-1000 ops)
- Performance benchmarks (daemon 2x faster than direct mode)

Results:
- Zero ID collisions across 1000+ concurrent creates
- All acceptance criteria validated for bd-110
- Create: 2.4ms (daemon) vs 4.7ms (direct)
- Update/List: similar 2x improvement

Tests Added:
- TestStressConcurrentAgents (8 agents, 800 creates)
- TestStressBatchOperations (4 agents, 400 batch ops)
- TestStressMixedOperations (6 agents, mixed read/write)
- TestStressNoUniqueConstraintViolations (10 agents, 1000 creates)
- BenchmarkDaemonCreate/Update/List/Latency
- Fixed flaky TestConcurrentRequests (shared client issue)

Files:
- internal/rpc/protocol.go - Added OpBatch, BatchArgs, BatchResponse
- internal/rpc/server.go - Implemented handleBatch with stop-on-failure
- internal/rpc/client.go - Added SetTimeout and Batch methods
- internal/rpc/stress_test.go - All stress tests
- internal/rpc/bench_test.go - Performance benchmarks
- DAEMON_STRESS_TEST.md - Complete documentation

Closes bd-114, bd-110

Amp-Thread-ID: https://ampcode.com/threads/T-1c07c140-0420-49fe-add1-b0b83b1bdff5
Co-authored-by: Amp <amp@ampcode.com>

internal/rpc/rpc_test.go

@@ -209,7 +209,7 @@ func TestSocketCleanup(t *testing.T) {
 }
 
 func TestConcurrentRequests(t *testing.T) {
-	_, client, cleanup := setupTestServer(t)
+	server, _, cleanup := setupTestServer(t)
 	defer cleanup()
 
 	done := make(chan bool)
@@ -217,6 +217,14 @@ func TestConcurrentRequests(t *testing.T) {
 
 	for i := 0; i < 5; i++ {
 		go func(n int) {
+			client, err := TryConnect(server.socketPath)
+			if err != nil {
+				errors <- err
+				done <- true
+				return
+			}
+			defer client.Close()
+
 			args := &CreateArgs{
 				Title:     "Concurrent Issue",
 				IssueType: "task",