fix(spawn): persist the resolved default agent on the session (#221)

A spawn with no explicit harness ran the daemon default (claude-code) but
stored an empty harness: effectiveHarness returned "", seedRecord persisted it,
and the empty->default resolution lived only inside agentRegistry.Agent. The API
then omitted harness and the UI defaulted to "codex" — mislabelling a Claude
Code session.

Inject the daemon's default agent (AO_AGENT / config.DefaultAgent) into the
session manager and resolve an unspecified harness to it before the seed row is
written, so the stored/returned harness matches the agent that actually runs.

Closes #220

backend/internal/daemon/lifecycle_wiring.go

@@ -59,7 +59,14 @@ func (l *lifecycleStack) Stop() {
 // store + LCM, the per-session agent resolver (AO_AGENT default), and the
 // agent messenger. The returned service is mounted at httpd APIDeps.Sessions.
 func startSession(cfg config.Config, runtime ports.Runtime, store *sqlite.Store, lcm *lifecycle.Manager, messenger ports.AgentMessenger, log *slog.Logger) (*sessionsvc.Service, error) {
-	agents, err := buildAgentResolver(cfg.Agent, log)
+	// Resolve the default agent once and share it with both the resolver (which
+	// launches it for an unspecified harness) and the session manager (which
+	// persists it onto the seed row), so the stored harness matches what runs.
+	defaultAgent := cfg.Agent
+	if defaultAgent == "" {
+		defaultAgent = config.DefaultAgent
+	}
+	agents, err := buildAgentResolver(defaultAgent, log)
 	if err != nil {
 		return nil, err
 	}
@@ -76,14 +83,15 @@ func startSession(cfg config.Config, runtime ports.Runtime, store *sqlite.Store,
 		return nil, fmt.Errorf("session workspace: %w", err)
 	}
 	mgr := sessionmanager.New(sessionmanager.Deps{
-		Runtime:   runtime,
-		Agents:    agents,
-		Workspace: ws,
-		Store:     store,
-		Messenger: messenger,
-		Lifecycle: lcm,
-		DataDir:   cfg.DataDir,
-		Logger:    log,
+		Runtime:        runtime,
+		Agents:         agents,
+		Workspace:      ws,
+		Store:          store,
+		Messenger:      messenger,
+		Lifecycle:      lcm,
+		DataDir:        cfg.DataDir,
+		DefaultHarness: domain.AgentHarness(defaultAgent),
+		Logger:         log,
 	})
 	scmProvider, err := newGitHubSCMProvider(log)
 	if err != nil {

backend/internal/session_manager/manager.go

@@ -82,7 +82,11 @@ type Manager struct {
 	messenger ports.AgentMessenger
 	lcm       lifecycleRecorder
 	dataDir   string
-	clock     func() time.Time
+	// defaultHarness is the daemon's configured default agent (AO_AGENT). A spawn
+	// that names no harness resolves to it before the seed row is written, so the
+	// stored/returned harness matches the agent the resolver actually launches.
+	defaultHarness domain.AgentHarness
+	clock          func() time.Time
 	// lookPath is exec.LookPath in production; tests substitute a stub so
 	// they don't need real binaries on PATH. Returns ports.ErrAgentBinaryNotFound
 	// when the binary is missing so the sentinel propagates through toAPIError.
@@ -105,7 +109,12 @@ type Deps struct {
 	// DataDir is exported to spawned agents as AO_DATA_DIR so their hook
 	// commands can open the same store.
 	DataDir string
-	Clock   func() time.Time
+	// DefaultHarness is the daemon's configured default agent (AO_AGENT), used to
+	// resolve a spawn that names no harness. Wiring passes config.DefaultAgent;
+	// left empty, an unspecified harness stays empty (the resolver still defaults
+	// it at launch, but the record won't reflect the real agent).
+	DefaultHarness domain.AgentHarness
+	Clock          func() time.Time
 	// LookPath overrides exec.LookPath for the pre-launch agent-binary check.
 	// Production wiring leaves this nil and the manager defaults to
 	// exec.LookPath; tests inject a stub so they need not seed real binaries.
@@ -123,17 +132,18 @@ type Deps struct {
 // time.Now when Deps.Clock is nil.
 func New(d Deps) *Manager {
 	m := &Manager{
-		runtime:    d.Runtime,
-		agents:     d.Agents,
-		workspace:  d.Workspace,
-		store:      d.Store,
-		messenger:  d.Messenger,
-		lcm:        d.Lifecycle,
-		dataDir:    d.DataDir,
-		clock:      d.Clock,
-		lookPath:   d.LookPath,
-		executable: d.Executable,
-		logger:     d.Logger,
+		runtime:        d.Runtime,
+		agents:         d.Agents,
+		workspace:      d.Workspace,
+		store:          d.Store,
+		messenger:      d.Messenger,
+		lcm:            d.Lifecycle,
+		dataDir:        d.DataDir,
+		defaultHarness: d.DefaultHarness,
+		clock:          d.Clock,
+		lookPath:       d.LookPath,
+		executable:     d.Executable,
+		logger:         d.Logger,
 	}
 	if m.clock == nil {
 		m.clock = time.Now
@@ -162,6 +172,13 @@ func (m *Manager) Spawn(ctx context.Context, cfg ports.SpawnConfig) (domain.Sess
 	// A per-project role override picks the harness when the spawn names none,
 	// so a project can default workers to one agent and orchestrators to another.
 	cfg.Harness = effectiveHarness(cfg.Harness, cfg.Kind, project.Config)
+	// Resolve an unspecified harness to the daemon default BEFORE the seed row is
+	// written, so the stored/returned harness matches the agent the resolver
+	// launches (otherwise a default-agent session persists an empty harness and
+	// the UI can't tell which agent is running).
+	if cfg.Harness == "" {
+		cfg.Harness = m.defaultHarness
+	}
 
 	prompt, systemPrompt, err := m.buildSpawnTexts(ctx, cfg)
 	if err != nil {

backend/internal/session_manager/manager_test.go

@@ -279,6 +279,34 @@ func TestSpawn_ResolvesProjectConfig(t *testing.T) {
 	}
 }
 
+// TestSpawn_PersistsResolvedDefaultHarness locks the fix for the mislabelled
+// agent: a spawn that names no harness must persist the daemon's default agent
+// (so the API/UI report what actually runs), while an explicit harness wins.
+func TestSpawn_PersistsResolvedDefaultHarness(t *testing.T) {
+	st := newFakeStore()
+	st.projects["mer"] = domain.ProjectRecord{ID: "mer"}
+	m := New(Deps{
+		Runtime: &fakeRuntime{}, Agents: fakeAgents{}, Workspace: &fakeWorkspace{}, Store: st,
+		Messenger: &fakeMessenger{}, Lifecycle: &fakeLCM{store: st},
+		LookPath:       func(string) (string, error) { return "/bin/true", nil },
+		DefaultHarness: domain.HarnessClaudeCode,
+	})
+
+	if _, err := m.Spawn(ctx, ports.SpawnConfig{ProjectID: "mer", Kind: domain.KindWorker}); err != nil {
+		t.Fatal(err)
+	}
+	if got := st.sessions["mer-1"].Harness; got != domain.HarnessClaudeCode {
+		t.Fatalf("unspecified harness = %q, want resolved default %q", got, domain.HarnessClaudeCode)
+	}
+
+	if _, err := m.Spawn(ctx, ports.SpawnConfig{ProjectID: "mer", Kind: domain.KindWorker, Harness: domain.HarnessCodex}); err != nil {
+		t.Fatal(err)
+	}
+	if got := st.sessions["mer-2"].Harness; got != domain.HarnessCodex {
+		t.Fatalf("explicit harness = %q, want %q", got, domain.HarnessCodex)
+	}
+}
+
 func TestSpawn_AssignsIDAndGoesIdle(t *testing.T) {
 	m, st, rt, _ := newManager()
 	s, err := m.Spawn(ctx, ports.SpawnConfig{ProjectID: "mer", Kind: domain.KindWorker, Prompt: "do it"})