fix: repair broken examples and graduate UV runtime from experimental

- hello-world-node: wrap server startup in async main() with proper
  await on server.connect(transport) — fixes crash on initialize in
  Claude Desktop UtilityProcess runtime (closes #200)
- hello-world-uv: add required mcp_config block — v0.4 schema requires
  it but the example omitted it, causing mcpb pack to fail
- file-manager-python: convert from broken v0.1/python to v0.4/uv —
  the old manifest had privacy_policies on v0.1 (schema violation),
  used command "python" (ENOENT on macOS), and had no bundled deps.
  Now uses UV runtime with proper mcp_config (closes #47, closes #72)
- Graduate UV runtime from experimental across docs and schema comments
- Bump DEFAULT_MANIFEST_VERSION from 0.2 to 0.3

All three examples verified: pack succeeds, server responds to MCP
initialize over stdio, tools/list returns expected tools.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/schemas_loose/0.4.ts

@@ -1,5 +1,4 @@
 // v0.4: Adds UV runtime support for Python extensions
-// NOTE: This schema version is experimental and subject to change.
 import * as z from "zod";
 
 export const MANIFEST_VERSION = "0.4";