Adds a new MCP tool that accepts base64-encoded file content and handles
the full attachment workflow (register → upload → return embedMarkdown).
This makes file uploads accessible to MCP clients like Hermes that can
only interact via registered tools.
Also updates the API reference resource with embedding format docs.
The upload response now includes an embedMarkdown field with the correct
Outline attachment syntax ([name size](/api/attachments.redirect?id=...))
so callers can insert it directly into documents for native card rendering.
IMPLEMENTATION.md updated with storage backend details, auth header,
and embedding format documentation.
b3nw