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>

scripts/build-mcpb-schema.js

@@ -1,6 +1,7 @@
 import { McpbManifestSchema as McpbManifestSchema_v0_1 } from "../dist/schemas/0.1.js";
 import { McpbManifestSchema as McpbManifestSchema_v0_2 } from "../dist/schemas/0.2.js";
 import { McpbManifestSchema as McpbManifestSchema_v0_3 } from "../dist/schemas/0.3.js";
+import { McpbManifestSchema as McpbManifestSchema_v0_4 } from "../dist/schemas/0.4.js";
 import { McpbSignatureInfoSchema } from "../dist/shared/common.js";
 import { zodToJsonSchema } from "zod-to-json-schema";
 import fs from "node:fs/promises";
@@ -14,6 +15,7 @@ const versionedManifestSchemas = {
   "mcpb-manifest-v0.1": McpbManifestSchema_v0_1,
   "mcpb-manifest-v0.2": McpbManifestSchema_v0_2,
   "mcpb-manifest-v0.3": McpbManifestSchema_v0_3,
+  "mcpb-manifest-v0.4": McpbManifestSchema_v0_4,
 };
 
 // Other schemas