Add skill for Himalaya (https://github.com/pimalaya/himalaya), a CLI
email client supporting IMAP, SMTP, Notmuch, and Sendmail backends.
Includes:
- SKILL.md with common operations (list, read, reply, forward, send)
- Configuration reference for Gmail, iCloud, and generic IMAP/SMTP
- MML (MIME Meta Language) composition guide for attachments
Tested with iCloud IMAP account - verified folder listing, email
reading, and sending work correctly.
