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>

examples/hello-world-uv/README.md

@@ -1,6 +1,4 @@
-# Hello World UV Runtime Example (Experimental)
-
-> **Note:** UV runtime support is experimental and may change in future versions.
+# Hello World UV Runtime Example
 
 This example demonstrates a minimal MCP server using **UV runtime**.
 
@@ -28,7 +26,7 @@ hello-world-uv/
 **UV Runtime** (this example):
 - `server.type = "uv"`
 - No bundled dependencies
-- No `mcp_config` needed
+- `mcp_config` uses `uv run` to auto-resolve deps from `pyproject.toml`
 - Small bundle size (~2 KB)
 - Works on any platform