fix(daemon): Kill zombie tmux sessions before recreating

The daemon was failing to restart agents when zombie tmux sessions existed
(session alive but Claude dead). Added EnsureSessionFresh() helper to
tmux package that:
- Checks if session exists
- If exists but Claude not running (zombie), kills the session
- Creates fresh session

Updated all daemon session creation points to use EnsureSessionFresh:
- ensureDeaconRunning()
- ensureWitnessRunning()
- restartPolecatSession()
- restartSession() in lifecycle.go

Added tests for the new helper function. (gt-j1i0r)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

internal/daemon/lifecycle.go

@@ -348,7 +348,8 @@ func (d *Daemon) restartSession(sessionName, identity string) error {
 	}
 
 	// Create session
-	if err := d.tmux.NewSession(sessionName, workDir); err != nil {
+	// Use EnsureSessionFresh to handle zombie sessions that exist but have dead Claude
+	if err := d.tmux.EnsureSessionFresh(sessionName, workDir); err != nil {
 		return fmt.Errorf("creating session: %w", err)
 	}