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),