v0.4.0

feat: MCP server (anvil mcp); bump to v0.4.0

Adds anvil.mcp_server with five tools:
  anvil_list_tasks, anvil_eval, anvil_manifest_diff,
  anvil_manifest_verify, anvil_doctor.

Transport: stdio (default, for Claude Desktop/Code) or --http
on localhost:8765. Install via pip install 'anvil-eval[mcp]'.

New CLI subcommand: anvil mcp [--http]
New optional extra: .[mcp] = mcp>=1.0
mypy overrides updated for mcp.* imports.