agent: serialize Submit with submitMu to prevent command/turn races

Commands like /compact arriving via ctl and prompts arriving via prompt
both call Core.Submit in separate goroutines. handleCommand for mutating
commands (compact, clear, agent, backend) modifies session state without
setting currentAction, so IsRunning() returns false and a concurrent
executeTurn can race.

Add submitMu held during the serialized path of Submit. The fast path
(inject/FIFO push while running) remains lock-free.

Also fix pre-existing race in TestSessionFileStoreWritePrompt: replace
time.Sleep synchronization with a channel signal.

pkg/agent/core.go

@@ -180,6 +180,7 @@ type agent struct {
 	env             map[string]string // session-scoped env vars
 	changeMu        sync.Mutex
 	changeCond      *sync.Cond
+	submitMu        sync.Mutex // serializes Submit calls (commands + turns)
 }
 
 // SetEnv stores a session-scoped variable and propagates it to the execute server.
@@ -673,6 +674,23 @@ func (s *agent) Submit(ctx context.Context, input string, handler EventHandler)
 	if input == "" {
 		return
 	}
+
+	// Fast path: inject and FIFO push use atomics and are safe without
+	// the submit lock. Handle them before acquiring submitMu so they
+	// don't block behind a long-running turn or command.
+	if s.IsRunning() {
+		if s.handleCommand(ctx, input, handler) {
+			return
+		}
+		s.fifo.Push(input)
+		return
+	}
+
+	// Serialize commands and turns so that e.g. a /compact arriving via
+	// ctl cannot race with an executeTurn arriving via prompt.
+	s.submitMu.Lock()
+	defer s.submitMu.Unlock()
+
 	if s.handleCommand(ctx, input, handler) {
 		return
 	}

pkg/store/store_test.go

@@ -41,6 +41,7 @@ type stubCore struct {
 	queued     []string
 	interrupted     bool
 	setSessionIDErr error
+	submitCh        chan struct{} // closed after each Submit call
 }
 
 func (c *stubCore) Submit(_ context.Context, input string, handler agent.EventHandler) {
@@ -48,6 +49,9 @@ func (c *stubCore) Submit(_ context.Context, input string, handler agent.EventHa
 	if handler != nil && c.reply != "" {
 		handler(agent.Event{Role: "assistant", Content: c.reply})
 	}
+	if c.submitCh != nil {
+		close(c.submitCh)
+	}
 }
 func (c *stubCore) Interrupt(error) bool                                        { c.interrupted = true; return c.running }
 func (c *stubCore) Inject(string)                                               {}
@@ -815,8 +819,9 @@ func TestSessionFileStoreWritePrompt(t *testing.T) {
 	defer sess.Cancel()
 	sf, core := newSessionFileStore(t, sess)
 
+	core.submitCh = make(chan struct{})
 	storeWrite(t, sf, "prompt", []byte("hello agent"))
-	time.Sleep(10 * time.Millisecond)
+	<-core.submitCh
 	if len(core.submitted) != 1 || core.submitted[0] != "hello agent" {
 		t.Errorf("submitted = %v; want [hello agent]", core.submitted)
 	}