feat(tmux): add KillSessionWithProcesses for explicit process termination

Before calling tmux kill-session, explicitly kill the pane's process tree
using pkill. This ensures claude processes don't survive session termination
due to SIGHUP being caught/ignored.

Implementation:
- Add KillSessionWithProcesses() to tmux.go
- Update killSessionsInOrder() in start.go to use new method
- Update stopSession() in down.go to use new method

Fixes: gt-5r7zr

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

internal/cmd/down.go

@@ -387,8 +387,8 @@ func stopSession(t *tmux.Tmux, sessionName string) (bool, error) {
 		time.Sleep(100 * time.Millisecond)
 	}
 
-	// Kill the session
-	return true, t.KillSession(sessionName)
+	// Kill the session (with explicit process termination to prevent orphans)
+	return true, t.KillSessionWithProcesses(sessionName)
 }
 
 // acquireShutdownLock prevents concurrent shutdowns.

internal/cmd/start.go

@@ -627,7 +627,7 @@ func killSessionsInOrder(t *tmux.Tmux, sessions []string, mayorSession, deaconSe
 
 	// 1. Stop Deacon first
 	if inList(deaconSession) {
-		if err := t.KillSession(deaconSession); err == nil {
+		if err := t.KillSessionWithProcesses(deaconSession); err == nil {
 			fmt.Printf("  %s %s stopped\n", style.Bold.Render("✓"), deaconSession)
 			stopped++
 		}
@@ -638,7 +638,7 @@ func killSessionsInOrder(t *tmux.Tmux, sessions []string, mayorSession, deaconSe
 		if sess == deaconSession || sess == mayorSession {
 			continue
 		}
-		if err := t.KillSession(sess); err == nil {
+		if err := t.KillSessionWithProcesses(sess); err == nil {
 			fmt.Printf("  %s %s stopped\n", style.Bold.Render("✓"), sess)
 			stopped++
 		}
@@ -646,7 +646,7 @@ func killSessionsInOrder(t *tmux.Tmux, sessions []string, mayorSession, deaconSe
 
 	// 3. Stop Mayor last
 	if inList(mayorSession) {
-		if err := t.KillSession(mayorSession); err == nil {
+		if err := t.KillSessionWithProcesses(mayorSession); err == nil {
 			fmt.Printf("  %s %s stopped\n", style.Bold.Render("✓"), mayorSession)
 			stopped++
 		}