refactor(cli): use standardized hydrateString for custom command prompts

This replaces the manual regex for ${extensionPath} with the standardized
hydrateString utility, automatically enabling support for other variables
like ${workspacePath} and ${/} in custom extension commands.

Added a test case to verify ${workspacePath} hydration.

packages/cli/src/services/FileCommandLoader.test.ts

@@ -955,6 +955,64 @@ describe('FileCommandLoader', () => {
         assert.fail('Incorrect action type');
       }
     });
+
+    it('correctly injects ${workspacePath} into extension commands', async () => {
+      const workspaceDir = process.cwd();
+      const extensionDir = path.join(
+        workspaceDir,
+        GEMINI_DIR,
+        'extensions',
+        'ws-test-ext',
+      );
+
+      mock({
+        [extensionDir]: {
+          'gemini-extension.json': JSON.stringify({
+            name: 'ws-test-ext',
+            version: '1.0.0',
+          }),
+          commands: {
+            'ws-cmd.toml': 'prompt = "WS: ${workspacePath}"',
+          },
+        },
+      });
+
+      const mockConfig = {
+        getProjectRoot: vi.fn(() => workspaceDir),
+        getExtensions: vi.fn(() => [
+          {
+            name: 'ws-test-ext',
+            version: '1.0.0',
+            isActive: true,
+            path: extensionDir,
+          },
+        ]),
+        getFolderTrust: vi.fn(() => false),
+        isTrustedFolder: vi.fn(() => false),
+      } as unknown as Config;
+      const loader = new FileCommandLoader(mockConfig);
+      const commands = await loader.loadCommands(signal);
+
+      expect(commands).toHaveLength(1);
+      const command = commands[0];
+
+      const result = await command.action?.(
+        createMockCommandContext({
+          invocation: {
+            raw: '/ws-cmd',
+            name: 'ws-cmd',
+            args: '',
+          },
+        }),
+        '',
+      );
+
+      if (result?.type === 'submit_prompt') {
+        expect(result.content).toEqual([{ text: `WS: ${workspaceDir}` }]);
+      } else {
+        assert.fail('Incorrect action type');
+      }
+    });
   });
 
   describe('Argument Handling Integration (via ShellProcessor)', () => {

packages/cli/src/services/FileCommandLoader.ts

@@ -33,6 +33,7 @@ import {
 } from './prompt-processors/shellProcessor.js';
 import { AtFileProcessor } from './prompt-processors/atFileProcessor.js';
 import { sanitizeForDisplay } from '../ui/utils/textUtils.js';
+import { hydrateString } from '../config/extensions/variables.js';
 
 interface CommandDirectory {
   path: string;
@@ -239,13 +240,13 @@ export class FileCommandLoader implements ICommandLoader {
 
     const validDef = validationResult.data;
 
-    // Hydrate extensionPath if this is an extension command
-    if (extensionPath) {
-      validDef.prompt = validDef.prompt.replace(
-        /\$\{extensionPath\}/g,
-        () => extensionPath,
-      );
-    }
+    // Hydrate variables in the prompt
+    validDef.prompt = hydrateString(validDef.prompt, {
+      extensionPath,
+      workspacePath: this.projectRoot,
+      '/': path.sep,
+      pathSeparator: path.sep,
+    });
 
     const relativePathWithExt = path.relative(baseDir, filePath);
     const relativePath = relativePathWithExt.substring(