fix(shutdown): kill entire process tree to prevent orphaned Claude processes

The previous implementation used `pkill -P pid` which only kills direct
children. When Claude spawns subprocesses (like node workers), those
grandchild processes would become orphaned (PPID=1) when their parent
was killed, causing them to survive `gt shutdown -fa`.

The fix recursively finds all descendant processes and kills them in
deepest-first order, ensuring no process becomes orphaned during
shutdown.

Fixes: gt-wd3ce

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/tmux/tmux.go

@@ -139,12 +139,13 @@ func (t *Tmux) KillSession(name string) error {
 //
 // Process:
 // 1. Get the pane's main process PID
-// 2. Send SIGTERM to all child processes (pkill -TERM -P <pid>)
-// 3. Wait 100ms for graceful shutdown
-// 4. Send SIGKILL to any remaining children (pkill -KILL -P <pid>)
-// 5. Kill the tmux session
+// 2. Find all descendant processes recursively (not just direct children)
+// 3. Send SIGTERM to all descendants (deepest first)
+// 4. Wait 100ms for graceful shutdown
+// 5. Send SIGKILL to any remaining descendants
+// 6. Kill the tmux session
 //
-// This ensures Claude processes are properly terminated even if they ignore SIGHUP.
+// This ensures Claude processes and all their children are properly terminated.
 func (t *Tmux) KillSessionWithProcesses(name string) error {
 	// Get the pane PID
 	pid, err := t.GetPanePID(name)
@@ -154,20 +155,49 @@ func (t *Tmux) KillSessionWithProcesses(name string) error {
 	}
 
 	if pid != "" {
-		// Send SIGTERM to child processes
-		_ = exec.Command("pkill", "-TERM", "-P", pid).Run()
+		// Get all descendant PIDs recursively (returns deepest-first order)
+		descendants := getAllDescendants(pid)
+
+		// Send SIGTERM to all descendants (deepest first to avoid orphaning)
+		for _, dpid := range descendants {
+			_ = exec.Command("kill", "-TERM", dpid).Run()
+		}
 
 		// Wait for graceful shutdown
 		time.Sleep(100 * time.Millisecond)
 
-		// Send SIGKILL to any remaining children
-		_ = exec.Command("pkill", "-KILL", "-P", pid).Run()
+		// Send SIGKILL to any remaining descendants
+		for _, dpid := range descendants {
+			_ = exec.Command("kill", "-KILL", dpid).Run()
+		}
 	}
 
 	// Kill the tmux session
 	return t.KillSession(name)
 }
 
+// getAllDescendants recursively finds all descendant PIDs of a process.
+// Returns PIDs in deepest-first order so killing them doesn't orphan grandchildren.
+func getAllDescendants(pid string) []string {
+	var result []string
+
+	// Get direct children using pgrep
+	out, err := exec.Command("pgrep", "-P", pid).Output()
+	if err != nil {
+		return result
+	}
+
+	children := strings.Fields(strings.TrimSpace(string(out)))
+	for _, child := range children {
+		// First add grandchildren (recursively) - deepest first
+		result = append(result, getAllDescendants(child)...)
+		// Then add this child
+		result = append(result, child)
+	}
+
+	return result
+}
+
 // KillServer terminates the entire tmux server and all sessions.
 func (t *Tmux) KillServer() error {
 	_, err := t.run("kill-server")

internal/tmux/tmux_test.go

@@ -527,3 +527,28 @@ func TestHasClaudeChild(t *testing.T) {
 		t.Error("hasClaudeChild should return false for nonexistent PID")
 	}
 }
+
+func TestGetAllDescendants(t *testing.T) {
+	// Test the getAllDescendants helper function
+
+	// Test with nonexistent PID - should return empty slice
+	got := getAllDescendants("999999999")
+	if len(got) != 0 {
+		t.Errorf("getAllDescendants(nonexistent) = %v, want empty slice", got)
+	}
+
+	// Test with PID 1 (init/launchd) - should find some descendants
+	// Note: We can't test exact PIDs, just that the function doesn't panic
+	// and returns reasonable results
+	descendants := getAllDescendants("1")
+	t.Logf("getAllDescendants(\"1\") found %d descendants", len(descendants))
+
+	// Verify returned PIDs are all numeric strings
+	for _, pid := range descendants {
+		for _, c := range pid {
+			if c < '0' || c > '9' {
+				t.Errorf("getAllDescendants returned non-numeric PID: %q", pid)
+			}
+		}
+	}
+}