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/CloneManager.tsx

@@ -15,7 +15,7 @@ interface CloneFormData {
   privacyOptIn: boolean;
 }
 
-const DEFAULT_WORKSPACE = '~/.openclaw/zclaw-workspace';
+const DEFAULT_WORKSPACE = '~/.openfang/zclaw-workspace';
 
 function createFormFromDraft(quickConfig: {
   agentName?: string;
@@ -119,7 +119,7 @@ export function CloneManager() {
     role: '默认助手',
     nickname: a.name,
     scenarios: [],
-    workspaceDir: '~/.openclaw/zclaw-workspace',
+    workspaceDir: '~/.openfang/zclaw-workspace',
     userName: quickConfig.userName || '未设置',
     userRole: '',
     restrictFiles: true,