fix(core): address extension context stickiness and symlink path resolution

This commit addresses two bugs identified during review:

1. Cleared the sticky `activeExtensionContext` when the standard `/plan` command is executed, ensuring subsequent prompts correctly target the default global plan directory.
2. Fixed a path resolution regression in `Storage.getPlansDir()` by constructing the fallback ENOENT path directly against the real project root. This prevents `isSubpath` validation failures and potential traversal vulnerabilities when the project root is a symlink.

packages/cli/src/test-utils/mockConfig.ts

@@ -23,6 +23,8 @@ export const createMockConfig = (overrides: Partial<Config> = {}): Config =>
     isInteractive: vi.fn(() => false),
     isInitialized: vi.fn(() => true),
     setTerminalBackground: vi.fn(),
+    setActiveExtensionContext: vi.fn(),
+    hasExtensionPlanDir: vi.fn(() => true),
     storage: {
       getProjectTempDir: vi.fn().mockReturnValue('/tmp/gemini-test'),
       initialize: vi.fn().mockResolvedValue(undefined),

packages/cli/src/ui/AppContainer.test.tsx

@@ -3182,6 +3182,39 @@ describe('AppContainer State Management', () => {
       );
       unmount();
     });
+
+    it('clears activeExtensionContext when /plan is explicitly executed', async () => {
+      const { checkPermissions } = await import(
+        './hooks/atCommandProcessor.js'
+      );
+      vi.mocked(checkPermissions).mockResolvedValue([]);
+
+      mockedUseSlashCommandProcessor.mockReturnValue({
+        handleSlashCommand: vi.fn(),
+        slashCommands: [{ name: 'plan', description: 'test', action: vi.fn() }],
+        pendingHistoryItems: [],
+        commandContext: {},
+        shellConfirmationRequest: null,
+        confirmationRequest: null,
+      });
+
+      const spySetActiveExtensionContext = vi.spyOn(
+        mockConfig,
+        'setActiveExtensionContext',
+      );
+
+      const { unmount } = await act(async () => renderAppContainer());
+
+      expect(capturedUIActions).toBeTruthy();
+
+      await act(async () =>
+        capturedUIActions.handleFinalSubmit('/plan my task'),
+      );
+
+      expect(spySetActiveExtensionContext).toHaveBeenCalledWith(undefined);
+
+      unmount();
+    });
   });
 
   describe('Overflow Hint Handling', () => {

packages/cli/src/ui/AppContainer.tsx

@@ -1313,9 +1313,13 @@ Logging in with Google... Restarting Gemini CLI to continue.
         slashCommands ?? [],
       );
 
-      if (parsedCommand.extensionContext && config) {
-        if (config.hasExtensionPlanDir(parsedCommand.extensionContext)) {
-          config.setActiveExtensionContext(parsedCommand.extensionContext);
+      if (config) {
+        if (parsedCommand.extensionContext) {
+          if (config.hasExtensionPlanDir(parsedCommand.extensionContext)) {
+            config.setActiveExtensionContext(parsedCommand.extensionContext);
+          }
+        } else if (parsedCommand.commandToExecute?.name === 'plan') {
+          config.setActiveExtensionContext(undefined);
         }
       }