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.