feat: network idle detection + chain pipe format

- Upgrade click/fill/select from domcontentloaded to networkidle wait
  (2s timeout, best-effort). Catches XHR/fetch triggered by interactions.
- Add pipe-delimited format to chain as JSON fallback:
  $B chain 'goto url | click @e5 | snapshot -ic'
- Add post-loop networkidle wait in chain when last command was a write.
- Frame-aware: commands use target (getActiveFrameOrPage) for locator ops,
  page-only ops (goto/back/forward/reload) guard against frame context.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

browse/test/commands.test.ts

@@ -1323,13 +1323,12 @@ describe('Errors', () => {
     }
   });
 
-  test('chain with invalid JSON throws', async () => {
-    try {
-      await handleMetaCommand('chain', ['not json'], bm, async () => {});
-      expect(true).toBe(false);
-    } catch (err: any) {
-      expect(err.message).toContain('Invalid JSON');
-    }
+  test('chain with invalid JSON falls back to pipe format', async () => {
+    // Non-JSON input is now treated as pipe-delimited format
+    // 'not json' → [["not", "json"]] → "not" is unknown command → error in result
+    const result = await handleMetaCommand('chain', ['not json'], bm, async () => {});
+    expect(result).toContain('ERROR');
+    expect(result).toContain('Unknown command: not');
   });
 
   test('chain with no arg throws', async () => {