feat(openfang): add bundled OpenFang runtime support

- Add prepare-openfang-runtime.mjs script for cross-platform binary download
- Update lib.rs to support binary runtime (fallback to Node.js for legacy)
- Add openfang.cmd/sh launcher scripts
- Update runtime-manifest.json for binary-based runtime
- Add README documentation for bundled runtime architecture

OpenFang binary is downloaded during build, supporting:
- Windows x64/ARM64 (.zip)
- macOS Intel/Apple Silicon (.tar.gz)
- Linux x64/ARM64 (.tar.gz)

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

desktop/src-tauri/resources/openfang-runtime/README.md

@@ -0,0 +1,76 @@
+# OpenFang Bundled Runtime
+
+This directory contains the bundled OpenFang runtime for ZClaw Desktop.
+
+## Architecture
+
+OpenFang is a **single Rust binary** (~32MB) that runs as the Agent OS backend.
+
+```
+openfang-runtime/
+├── openfang.exe                  # Windows binary
+├── openfang-x86_64-unknown-linux-gnu    # Linux x64 binary
+├── openfang-aarch64-unknown-linux-gnu   # Linux ARM64 binary
+├── openfang-x86_64-apple-darwin         # macOS Intel binary
+├── openfang-aarch64-apple-darwin        # macOS Apple Silicon binary
+├── runtime-manifest.json         # Runtime metadata
+├── openfang.cmd                  # Windows launcher
+├── openfang.sh                   # Unix launcher
+├── download-openfang.ps1         # Windows download script
+└── download-openfang.sh          # Unix download script
+```
+
+## Setup
+
+### Option 1: Download Binary
+
+**Windows (PowerShell):**
+```powershell
+cd desktop/src-tauri/resources/openfang-runtime
+.\download-openfang.ps1
+```
+
+**Linux/macOS:**
+```bash
+cd desktop/src-tauri/resources/openfang-runtime
+chmod +x download-openfang.sh
+./download-openfang.sh
+```
+
+### Option 2: Manual Download
+
+1. Go to https://github.com/RightNow-AI/openfang/releases
+2. Download the appropriate binary for your platform
+3. Place it in this directory
+
+## Build Integration
+
+The Tauri build process will include this directory in the application bundle:
+
+```json
+// tauri.conf.json
+{
+  "bundle": {
+    "resources": ["resources/openfang-runtime/"]
+  }
+}
+```
+
+## Runtime Resolution
+
+ZClaw Desktop resolves the OpenFang runtime in this order:
+
+1. `ZCLAW_OPENFANG_BIN` environment variable (for development)
+2. Bundled `openfang-runtime/` directory
+3. System PATH (`openfang`)
+
+## Endpoints
+
+- **WebSocket**: `ws://127.0.0.1:4200/ws`
+- **REST API**: `http://127.0.0.1:4200/api`
+
+## Version Info
+
+- OpenFang Version: 2026.3.13
+- Port: 4200 (was 18789 for OpenClaw)
+- Config: `~/.openfang/openfang.toml` (was `~/.openclaw/openclaw.json`)