Add startup connection validation to GUI

- Add validate_token() method to RegistryClient
  - Returns (is_valid, error_message) tuple
  - Catches 401/UNAUTHORIZED errors and connection issues
- Add startup validation in MainWindow
  - Runs 500ms after window shows (non-blocking)
  - Clears invalid tokens automatically
  - Shows status bar message explaining what happened
- Document cmdforge config disconnect command

This prevents confusing errors when trying to publish with stale or
revoked credentials - the GUI now auto-clears bad connections on restart.

Co-Authored-By: Claude Opus 4.5 <[email protected]>

src/cmdforge/gui/main_window.py

@@ -5,7 +5,7 @@ from PySide6.QtWidgets import (
     QListWidget, QListWidgetItem, QStackedWidget,
     QStatusBar, QLabel, QSplitter
 )
-from PySide6.QtCore import Qt, QSize, QSettings
+from PySide6.QtCore import Qt, QSize, QSettings, QTimer
 from PySide6.QtGui import QIcon, QFont, QShortcut, QKeySequence
 
 from .styles import STYLESHEET
@@ -68,6 +68,33 @@ class MainWindow(QMainWindow):
         # Restore window geometry
         self._restore_geometry()
 
+        # Validate registry connection after window shows
+        QTimer.singleShot(500, self._validate_registry_connection)
+
+    def _validate_registry_connection(self):
+        """Check if registry connection is valid on startup, clear if invalid."""
+        try:
+            from ..registry_client import RegistryClient
+            from ..config import set_registry_token, load_config
+
+            config = load_config()
+            if not config.registry.token:
+                return  # No connection configured, nothing to validate
+
+            client = RegistryClient()
+            is_valid, error = client.validate_token()
+
+            if not is_valid:
+                # Token is invalid, clear it
+                set_registry_token(None)
+                self.status_bar.showMessage(
+                    f"Registry connection cleared: {error}. Please reconnect.",
+                    10000  # Show for 10 seconds
+                )
+        except Exception as e:
+            # Don't crash on validation errors, just log to status
+            self.status_bar.showMessage(f"Could not validate registry connection: {e}", 5000)
+
     def _setup_sidebar(self):
         """Set up sidebar navigation items."""
         items = [