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.