fix(tmux): use KillSessionWithProcesses to prevent zombie bash processes

When Claude sessions were terminated using KillSession(), bash subprocesses
spawned by Claude's Bash tool could survive because they ignore SIGHUP.
This caused zombie processes to accumulate over time.

Changed all critical session termination paths to use KillSessionWithProcesses()
which explicitly kills all descendant processes before terminating the session.

Fixes: gt-ew3tk

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

internal/connection/connection.go

@@ -62,6 +62,7 @@ type Connection interface {
 	TmuxNewSession(name, dir string) error
 
 	// TmuxKillSession terminates the named tmux session.
+	// Uses KillSessionWithProcesses internally to ensure all descendant processes are killed.
 	TmuxKillSession(name string) error
 
 	// TmuxSendKeys sends keys to the named tmux session.