fix(witness): Kill tmux session on Stop()

The witness manager's Stop() method was only updating runtime JSON state
without killing the tmux session, causing 'gt rig shutdown' to leave
witness sessions running.

Added sessionName() method and tmux kill-session logic to match the
refinery's existing implementation.

Fixes: bd-gxaf

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

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

internal/witness/manager.go

@@ -2,11 +2,13 @@ package witness
 
 import (
 	"errors"
+	"fmt"
 	"os"
 	"time"
 
 	"github.com/steveyegge/gastown/internal/agent"
 	"github.com/steveyegge/gastown/internal/rig"
+	"github.com/steveyegge/gastown/internal/tmux"
 	"github.com/steveyegge/gastown/internal/util"
 )
 
@@ -52,6 +54,11 @@ func (m *Manager) saveState(w *Witness) error {
 	return m.stateManager.Save(w)
 }
 
+// sessionName returns the tmux session name for this witness.
+func (m *Manager) sessionName() string {
+	return fmt.Sprintf("gt-%s-witness", m.rig.Name)
+}
+
 // Status returns the current witness status.
 // ZFC-compliant: trusts agent-reported state, no PID inference.
 // The daemon reads agent bead state for liveness checks.
@@ -95,12 +102,23 @@ func (m *Manager) Stop() error {
 		return err
 	}
 
-	if w.State != StateRunning {
+	// Check if tmux session exists
+	t := tmux.NewTmux()
+	sessionID := m.sessionName()
+	sessionRunning, _ := t.HasSession(sessionID)
+
+	// If neither state nor session indicates running, it's not running
+	if w.State != StateRunning && !sessionRunning {
 		return ErrNotRunning
 	}
 
+	// Kill tmux session if it exists (best-effort: may already be dead)
+	if sessionRunning {
+		_ = t.KillSession(sessionID)
+	}
+
 	// If we have a PID, try to stop it gracefully
-	if w.PID > 0 && w.PID != os.Getpid() {
+	if w.PID > 0 && w.PID != os.Getpid() && util.ProcessExists(w.PID) {
 		// Send SIGTERM (best-effort graceful stop)
 		if proc, err := os.FindProcess(w.PID); err == nil {
 			_ = proc.Signal(os.Interrupt)