refactor(phase3): remove OpenClaw compatibility layer

Phase 3 Configuration Migration completed:
- Remove OpenClaw backend type selector from General.tsx
- Update default workspace path from ~/.openclaw to ~/.openfang
- Update default port from 18789 to 50051
- Archive openclaw.default.json to docs/archive/openclaw-legacy/
- Remove OpenClaw migration UI from Workspace settings
- Update About.tsx to reference OpenFang
- Clean up gateway-client.ts comments

All OpenClaw compatibility code removed. ZCLAW now exclusively
targets OpenFang (Rust Agent OS) backend.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

desktop/src/components/Settings/ModelsAPI.tsx

@@ -39,7 +39,7 @@ export function ModelsAPI() {
   const handleReconnect = () => {
     disconnect();
     setTimeout(() => connect(
-      gatewayUrl || quickConfig.gatewayUrl || 'ws://127.0.0.1:18789',
+      gatewayUrl || quickConfig.gatewayUrl || 'ws://127.0.0.1:50051/ws',
       gatewayToken || quickConfig.gatewayToken || getStoredGatewayToken()
     ).catch(() => {}), 500);
   };