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/core/src/config/storage.test.ts

@@ -358,6 +358,26 @@ describe('Storage – additional helpers', () => {
         expectedError:
           "Custom plans directory 'symlink-to-outside' resolves to '/outside/project/root', which is outside the project root '/tmp/project'.",
       },
+      {
+        name: 'non-existent plan dir in a symlinked project root',
+        customDir: 'new-plans',
+        setup: () => {
+          vi.mocked(fs.realpathSync).mockImplementation((p: fs.PathLike) => {
+            const pStr = p.toString();
+            if (pStr === projectRoot) {
+              return '/private/tmp/project';
+            }
+            if (pStr.includes('new-plans')) {
+              const err = new Error('ENOENT') as NodeJS.ErrnoException;
+              err.code = 'ENOENT';
+              throw err;
+            }
+            return pStr;
+          });
+          return () => vi.mocked(fs.realpathSync).mockRestore();
+        },
+        expected: path.resolve(projectRoot, 'new-plans'),
+      },
     ];
 
     testCases.forEach(({ name, customDir, expected, expectedError, setup }) => {

packages/core/src/config/storage.ts

@@ -333,6 +333,8 @@ export class Storage {
         ) {
           throw e;
         }
+        // Construct the fallback path safely against the real project root
+        realResolvedPath = path.resolve(realProjectRoot, customDir);
       }
 
       if (!isSubpath(realProjectRoot, realResolvedPath)) {