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>

examples/hello-world-uv/README.md

@@ -0,0 +1,62 @@
+# Hello World UV Runtime Example (Experimental)
+
+> **Note:** UV runtime support is experimental and may change in future versions.
+
+This example demonstrates a minimal MCP server using **UV runtime**.
+
+## What is UV Runtime?
+
+UV runtime lets Claude Desktop automatically manage Python and dependencies for your extension:
+- Downloads the correct Python version for the user's platform
+- Creates an isolated virtual environment
+- Installs dependencies from `pyproject.toml`
+- Works cross-platform (Windows, macOS, Linux) without user setup
+
+## Structure
+
+```
+hello-world-uv/
+├── manifest.json       # server.type = "uv"
+├── pyproject.toml      # Dependencies listed here
+├── .mcpbignore        # Exclude build artifacts
+└── src/
+    └── server.py       # MCP server implementation
+```
+
+## Key Differences from Python Runtime
+
+**UV Runtime** (this example):
+- `server.type = "uv"`
+- No bundled dependencies
+- No `mcp_config` needed
+- Small bundle size (~2 KB)
+- Works on any platform
+
+**Python Runtime** (traditional):
+- `server.type = "python"`
+- Must bundle dependencies in `server/lib/`
+- Requires `mcp_config` with PYTHONPATH
+- Larger bundle size
+- Only works with pure Python (no compiled deps)
+
+## Installing
+
+```bash
+mcpb pack
+```
+
+Install the generated `.mcpb` file in Claude Desktop.
+
+## Testing Locally
+
+```bash
+# Install dependencies
+uv sync
+
+# Run server
+uv run src/server.py
+```
+
+## Tools
+
+- **say_hello** - Greets a person by name