feat(mcp): add Hy3 email and message assistant

[Kelsey050218]

Summary

This PR adds a local stdio MCP server that uses Hy3 to process exported email and message files.

The server supports a complete inbox workflow:

• load .eml, .json, .jsonl, .txt, and .md files
• classify messages by category and priority
• extract action items, owners, deadlines, and unresolved questions
• draft replies without sending them
• generate a concise inbox digest

MCP tools

• load_messages
• triage_messages_with_hy3
• extract_action_items_with_hy3
• draft_replies_with_hy3
• generate_inbox_digest_with_hy3

All tools and parameters include MCP descriptions. Hy3 reasoning supports no_think, low, and high.

Safety

• reads only paths inside MESSAGE_ASSISTANT_ALLOWED_ROOTS
• never sends, deletes, or modifies messages
• requires HY3_API_KEY through the environment
• flags uncertain details instead of inventing deadlines, commitments, approvals, or prices

Packaging and client setup

• one-command local install with ./scripts/install.sh
• project-level Cursor configuration example
• project-level CodeBuddy configuration and CLI example
• English and Chinese README files
• JSON and EML demo inboxes

Validation

• 9 unit tests pass
• MCP protocol initialization succeeds
• all 5 tools can be listed and called over stdio
• tool and parameter descriptions are verified
• package installation succeeds in a new virtual environment
• configuration and sample JSON files pass syntax validation

Remaining acceptance evidence

• run the workflow against an available Hy3 endpoint
• capture calls in Cursor and CodeBuddy
• add the final demo GIF or video

Related to #3.