This implements task 9 from AGENT.md by adding:

- Flag color support with RGB bit pattern encoding (Red, Orange, Yellow, Green, Blue, Purple, Gray)
- Standard message keywords as defined in draft-ietf-mailmaint-messageflag-mailboxattribute-02
- Mailbox attribute names (Snoozed, Scheduled, Memos)
- Helper functions to work with these types and convert between typed and string representations
- Utility functions to check, add, and search for messages with specific keywords

Added a test program (flag_color_test) to demonstrate the new functionality.

🤖 Generated with [Claude Code](https://claude.ai/code)
Co-Authored-By: Claude <noreply@anthropic.com>