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/runtime-manifest.json

@@ -0,0 +1,20 @@
+{
+  "source": {
+    "binPath": "openfang.exe",
+    "binPathLinux": "openfang-x86_64-unknown-linux-gnu",
+    "binPathMac": "openfang-x86_64-apple-darwin",
+    "binPathMacArm": "openfang-aarch64-apple-darwin"
+  },
+  "stagedAt": "2026-03-13T09:08:38.514Z",
+  "version": "2026.03.13",
+  "runtimeType": "openfang",
+  "description": "OpenFang Agent OS - Single binary runtime (~32MB)",
+  "endpoints": {
+    "websocket": "ws://127.0.0.1:4200/ws",
+    "rest": "http://127.0.0.1:4200/api"
+  },
+  "platform": {
+    "os": "win32",
+    "arch": "x64"
+  }
+}