fix: wait for gateway ready before post-install model validation

Root cause: gateway restart is async (systemd), but validateNoReplyModelAvailable()
ran immediately after, hitting a race condition where the new gateway process
hadn't finished initializing yet. This caused 'model not listed' validation
failures, triggering config rollback even though the config was correct.

Changes:
- Add waitForGatewayReady() that polls 'openclaw gateway status' for RPC probe
- Add retry loop (5 attempts, 2s interval) to validateNoReplyModelAvailable()
- Fix CONFIG.example.json: contextWindow 4096->200000, maxTokens 64->8192
  (OpenClaw requires minimum 16000 contextWindow)

docs/CONFIG.example.json

@@ -40,8 +40,8 @@
             "reasoning": false,
             "input": ["text"],
             "cost": { "input": 0, "output": 0, "cacheRead": 0, "cacheWrite": 0 },
-            "contextWindow": 4096,
-            "maxTokens": 64
+            "contextWindow": 200000,
+            "maxTokens": 8192
           }
         ]
       }