feat: integrate A2A listener into Forever Mode

- Port external-listener.ts from st/a2a-listen branch
- Auto-start A2A HTTP server when Forever Mode + Sisyphus enabled
- Add a2aPort config to SisyphusModeSettings (GEMINI.md frontmatter)
- Bridge external messages into session and capture responses
- Display A2A port in status bar alongside Sisyphus timer
- Add ExternalMessage and A2AListenerStarted app events

packages/cli/src/config/config.ts

@@ -575,6 +575,10 @@ export async function loadCliConfig(
                 typeof sisyphusSettings['prompt'] === 'string'
                   ? sisyphusSettings['prompt']
                   : undefined,
+              a2aPort:
+                typeof sisyphusSettings['a2aPort'] === 'number'
+                  ? sisyphusSettings['a2aPort']
+                  : undefined,
             };
           }
         }