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>

config/openclaw.default.json

@@ -1,70 +0,0 @@
-{
-  "_comment": "ZCLAW default OpenClaw configuration",
-  "agents": {
-    "defaults": {
-      "workspace": "~/.openclaw/zclaw-workspace",
-      "model": {
-        "primary": "zhipu/glm-5",
-        "fallbacks": ["openai/gpt-4o", "qwen/qwen3.5-plus"]
-      },
-      "models": {
-        "zhipu/glm-5": { "alias": "GLM-5" },
-        "zhipu/glm-4.7": { "alias": "GLM-4.7" },
-        "qwen/qwen3.5-plus": { "alias": "Qwen3.5+" },
-        "kimi/kimi-k2.5": { "alias": "Kimi-K2.5" },
-        "minimax/minimax-m2.5": { "alias": "MiniMax-M2.5" },
-        "openai/gpt-4o": { "alias": "GPT-4o" }
-      },
-      "heartbeat": {
-        "every": "1h"
-      },
-      "sandbox": {
-        "workspaceRoot": "~/.openclaw/zclaw-workspace"
-      }
-    },
-    "list": [
-      {
-        "id": "main",
-        "groupChat": {
-          "mentionPatterns": ["@zclaw", "zclaw", "小龙虾"]
-        }
-      }
-    ]
-  },
-  "channels": {
-    "feishu": {
-      "enabled": false,
-      "accounts": {
-        "default": {
-          "appId": "",
-          "appSecret": "",
-          "enabled": false
-        }
-      }
-    }
-  },
-  "plugins": {
-    "load": {
-      "paths": [
-        "./plugins/zclaw-chinese-models",
-        "./plugins/zclaw-feishu",
-        "./plugins/zclaw-ui"
-      ]
-    }
-  },
-  "skills": {
-    "load": {
-      "extraDirs": ["./skills"]
-    }
-  },
-  "tools": {
-    "exec": {
-      "shell": true
-    },
-    "web": {
-      "search": {
-        "enabled": true
-      }
-    }
-  }
-}