feat: add UV runtime support in manifest v0.4

Add UV as a new server type for Python extensions that use UV runtime
for dependency management instead of bundling dependencies.

Schema changes (v0.4):
- Add "uv" to server.type enum
- Make mcp_config optional (not needed for UV type)
- UV extensions rely on pyproject.toml for dependencies

Documentation:
- Add UV runtime section to MANIFEST.md explaining benefits and usage
- Update README.md to recommend UV runtime for Python
- Add complete hello-world-uv example

Benefits of UV runtime:
- Cross-platform support (Windows/macOS/Linux, Intel/ARM)
- Small bundle size (~100 KB vs 5-10 MB)
- Handles compiled dependencies portably
- No user Python installation required

Example manifest:
{
  "manifest_version": "0.4",
  "server": {
    "type": "uv",
    "entry_point": "src/server.py"
  }
}

Co-authored-by: Joan <joan@anthropic.com>

README.md

@@ -109,12 +109,22 @@ bundle.mcpb (ZIP file)
 
 ### Bundling Dependencies
 
-**Python Bundles:**
+**UV Runtime (Experimental - v0.4+):**
 
+- Use `server.type = "uv"` in manifest
+- Include `pyproject.toml` with dependencies (no bundled packages needed)
+- Host application manages Python and dependencies automatically
+- Works cross-platform without user Python installation
+- See `examples/hello-world-uv`
+
+**Python Bundles (Traditional):**
+
+- Use `server.type = "python"` in manifest
 - Bundle all required packages in `server/lib/` directory
 - OR bundle a complete virtual environment in `server/venv/`
 - Use tools like `pip-tools`, `poetry`, or `pipenv` to create reproducible bundles
 - Set `PYTHONPATH` to include bundled packages via `mcp_config.env`
+- **Limitation**: Cannot portably bundle compiled dependencies (e.g., pydantic, which the MCP Python SDK requires)
 
 **Node.js Bundles:**