fix(tmux): prevent gt done from killing itself during session cleanup (#821)

When gt done runs inside a tmux session (e.g., after polecat task
completion), calling KillSessionWithProcesses would kill the gt done
process itself before it could complete cleanup operations like writing
handoff state.

Add KillSessionWithProcessesExcluding() function that accepts a list of
PIDs to exclude from the kill sequence. Update selfKillSession to pass
its own PID, ensuring gt done completes before the session is destroyed.

Also fix both Kill*WithProcesses functions to ignore "session not found"
errors from KillSession - when we kill all processes in a session, tmux
may automatically destroy it before we explicitly call KillSession.

Co-authored-by: julianknutsen <julianknutsen@users.noreply.github>
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

internal/tmux/tmux_test.go

@@ -554,6 +554,157 @@ func TestGetAllDescendants(t *testing.T) {
 	}
 }
 
+func TestKillSessionWithProcesses(t *testing.T) {
+	if !hasTmux() {
+		t.Skip("tmux not installed")
+	}
+
+	tm := NewTmux()
+	sessionName := "gt-test-killproc-" + t.Name()
+
+	// Clean up any existing session
+	_ = tm.KillSession(sessionName)
+
+	// Create session with a long-running process
+	cmd := `sleep 300`
+	if err := tm.NewSessionWithCommand(sessionName, "", cmd); err != nil {
+		t.Fatalf("NewSessionWithCommand: %v", err)
+	}
+
+	// Verify session exists
+	has, err := tm.HasSession(sessionName)
+	if err != nil {
+		t.Fatalf("HasSession: %v", err)
+	}
+	if !has {
+		t.Fatal("expected session to exist after creation")
+	}
+
+	// Kill with processes
+	if err := tm.KillSessionWithProcesses(sessionName); err != nil {
+		t.Fatalf("KillSessionWithProcesses: %v", err)
+	}
+
+	// Verify session is gone
+	has, err = tm.HasSession(sessionName)
+	if err != nil {
+		t.Fatalf("HasSession after kill: %v", err)
+	}
+	if has {
+		t.Error("expected session to not exist after KillSessionWithProcesses")
+		_ = tm.KillSession(sessionName) // cleanup
+	}
+}
+
+func TestKillSessionWithProcesses_NonexistentSession(t *testing.T) {
+	if !hasTmux() {
+		t.Skip("tmux not installed")
+	}
+
+	tm := NewTmux()
+
+	// Killing nonexistent session should not panic, just return error or nil
+	err := tm.KillSessionWithProcesses("nonexistent-session-xyz-12345")
+	// We don't care about the error value, just that it doesn't panic
+	_ = err
+}
+
+func TestKillSessionWithProcessesExcluding(t *testing.T) {
+	if !hasTmux() {
+		t.Skip("tmux not installed")
+	}
+
+	tm := NewTmux()
+	sessionName := "gt-test-killexcl-" + t.Name()
+
+	// Clean up any existing session
+	_ = tm.KillSession(sessionName)
+
+	// Create session with a long-running process
+	cmd := `sleep 300`
+	if err := tm.NewSessionWithCommand(sessionName, "", cmd); err != nil {
+		t.Fatalf("NewSessionWithCommand: %v", err)
+	}
+
+	// Verify session exists
+	has, err := tm.HasSession(sessionName)
+	if err != nil {
+		t.Fatalf("HasSession: %v", err)
+	}
+	if !has {
+		t.Fatal("expected session to exist after creation")
+	}
+
+	// Kill with empty excludePIDs (should behave like KillSessionWithProcesses)
+	if err := tm.KillSessionWithProcessesExcluding(sessionName, nil); err != nil {
+		t.Fatalf("KillSessionWithProcessesExcluding: %v", err)
+	}
+
+	// Verify session is gone
+	has, err = tm.HasSession(sessionName)
+	if err != nil {
+		t.Fatalf("HasSession after kill: %v", err)
+	}
+	if has {
+		t.Error("expected session to not exist after KillSessionWithProcessesExcluding")
+		_ = tm.KillSession(sessionName) // cleanup
+	}
+}
+
+func TestKillSessionWithProcessesExcluding_WithExcludePID(t *testing.T) {
+	if !hasTmux() {
+		t.Skip("tmux not installed")
+	}
+
+	tm := NewTmux()
+	sessionName := "gt-test-killexcl2-" + t.Name()
+
+	// Clean up any existing session
+	_ = tm.KillSession(sessionName)
+
+	// Create session with a long-running process
+	cmd := `sleep 300`
+	if err := tm.NewSessionWithCommand(sessionName, "", cmd); err != nil {
+		t.Fatalf("NewSessionWithCommand: %v", err)
+	}
+	defer func() { _ = tm.KillSession(sessionName) }()
+
+	// Get the pane PID
+	panePID, err := tm.GetPanePID(sessionName)
+	if err != nil {
+		t.Fatalf("GetPanePID: %v", err)
+	}
+	if panePID == "" {
+		t.Skip("could not get pane PID")
+	}
+
+	// Kill with the pane PID excluded - the function should still kill the session
+	// but should not kill the excluded PID before the session is destroyed
+	err = tm.KillSessionWithProcessesExcluding(sessionName, []string{panePID})
+	if err != nil {
+		t.Fatalf("KillSessionWithProcessesExcluding: %v", err)
+	}
+
+	// Session should be gone (the final KillSession always happens)
+	has, _ := tm.HasSession(sessionName)
+	if has {
+		t.Error("expected session to not exist after KillSessionWithProcessesExcluding")
+	}
+}
+
+func TestKillSessionWithProcessesExcluding_NonexistentSession(t *testing.T) {
+	if !hasTmux() {
+		t.Skip("tmux not installed")
+	}
+
+	tm := NewTmux()
+
+	// Killing nonexistent session should not panic
+	err := tm.KillSessionWithProcessesExcluding("nonexistent-session-xyz-12345", []string{"12345"})
+	// We don't care about the error value, just that it doesn't panic
+	_ = err
+}
+
 func TestSessionSet(t *testing.T) {
 	if !hasTmux() {
 		t.Skip("tmux not installed")