Adds docs/superpowers/specs/2026-06-30-files-mcp-design.md describing a
sibling FastAPI sub-app (mount: /files-mcp) that exposes 9 file operations
(create/read/update/delete/list_dir/stat/search/move/copy) over fastapi-mcp,
sandboxed to FILES_MCP_ROOT with utf-8 text only and atomic writes.
Branch: feat/files-mcp
Mount path: /files-mcp
New package: files_mcp/
Backward compat: 0 existing tools / tests / dependencies changed.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
