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/.gitignore

@@ -11,6 +11,16 @@ node_modules
 dist
 dist-ssr
 *.local
+src-tauri/resources/openclaw-runtime/*
+!src-tauri/resources/openclaw-runtime/.gitkeep
+local-tools/*
+!local-tools/.gitkeep
+!local-tools/NSIS/
+!local-tools/NSIS/.gitkeep
+!local-tools/WixTools/
+!local-tools/WixTools/.gitkeep
+installer-smoke/
+msi-smoke/
 
 # Editor directories and files
 .vscode/*
@@ -22,3 +32,4 @@ dist-ssr
 *.njsproj
 *.sln
 *.sw?
+desktop/src-tauri/resources/openfang-runtime/openfang.exe