fix(tools): use sync.Once for thread-safe Stop() in SessionManager

The Stop() method previously used a select/default pattern which was not
safe under concurrent calls — two goroutines could both pass the check
and attempt to close the same channel, causing a panic.

Replace with sync.Once to guarantee exactly-once close semantics,
matching the documented contract of being safe for concurrent use.

Review feedback: afjcjsbx

pkg/tools/session.go

@@ -172,6 +172,7 @@ type SessionManager struct {
 	mu       sync.RWMutex
 	sessions map[string]*ProcessSession
 	stopCh   chan struct{}
+	stopOnce sync.Once
 }
 
 func NewSessionManager() *SessionManager {
@@ -197,16 +198,13 @@ func NewSessionManager() *SessionManager {
 	return sm
 }
 
-// Stop shuts down the background cleanup goroutine. Safe to call multiple times.
-// After Stop returns, the SessionManager is still usable — only the cleanup
-// goroutine is terminated.
+// Stop shuts down the background cleanup goroutine. Safe to call multiple
+// times from concurrent goroutines. After Stop returns, the SessionManager
+// is still usable — only the cleanup goroutine is terminated.
 func (sm *SessionManager) Stop() {
-	select {
-	case <-sm.stopCh:
-		// already closed
-	default:
+	sm.stopOnce.Do(func() {
 		close(sm.stopCh)
-	}
+	})
 }
 
 // cleanupOldSessions removes sessions that are done and older than 30 minutes

pkg/tools/shell_test.go

@@ -1,4 +1,4 @@
-package tools
+package tools
 
 import (
 	"context"