feat: add himalaya email CLI skill

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.

skills/himalaya/references/configuration.md

@@ -0,0 +1,175 @@
+# Himalaya Configuration Reference
+
+Configuration file location: `~/.config/himalaya/config.toml`
+
+## Minimal IMAP + SMTP Setup
+
+```toml
+[accounts.default]
+email = "user@example.com"
+display-name = "Your Name"
+default = true
+
+# IMAP backend for reading emails
+backend.type = "imap"
+backend.host = "imap.example.com"
+backend.port = 993
+backend.encryption.type = "tls"
+backend.login = "user@example.com"
+backend.auth.type = "password"
+backend.auth.raw = "your-password"
+
+# SMTP backend for sending emails
+message.send.backend.type = "smtp"
+message.send.backend.host = "smtp.example.com"
+message.send.backend.port = 587
+message.send.backend.encryption.type = "start-tls"
+message.send.backend.login = "user@example.com"
+message.send.backend.auth.type = "password"
+message.send.backend.auth.raw = "your-password"
+```
+
+## Password Options
+
+### Raw password (testing only, not recommended)
+```toml
+backend.auth.raw = "your-password"
+```
+
+### Password from command (recommended)
+```toml
+backend.auth.cmd = "pass show email/imap"
+backend.auth.cmd = "security find-generic-password -a user@example.com -s imap -w"
+```
+
+### System keyring (requires keyring feature)
+```toml
+backend.auth.keyring = "imap-example"
+```
+Then run `himalaya configure -a <account>` to store the password.
+
+## Gmail Configuration
+
+```toml
+[accounts.gmail]
+email = "you@gmail.com"
+display-name = "Your Name"
+default = true
+
+backend.type = "imap"
+backend.host = "imap.gmail.com"
+backend.port = 993
+backend.encryption.type = "tls"
+backend.login = "you@gmail.com"
+backend.auth.type = "password"
+backend.auth.cmd = "pass show google/app-password"
+
+message.send.backend.type = "smtp"
+message.send.backend.host = "smtp.gmail.com"
+message.send.backend.port = 587
+message.send.backend.encryption.type = "start-tls"
+message.send.backend.login = "you@gmail.com"
+message.send.backend.auth.type = "password"
+message.send.backend.auth.cmd = "pass show google/app-password"
+```
+
+**Note:** Gmail requires an App Password if 2FA is enabled.
+
+## iCloud Configuration
+
+```toml
+[accounts.icloud]
+email = "you@icloud.com"
+display-name = "Your Name"
+
+backend.type = "imap"
+backend.host = "imap.mail.me.com"
+backend.port = 993
+backend.encryption.type = "tls"
+backend.login = "you@icloud.com"
+backend.auth.type = "password"
+backend.auth.cmd = "pass show icloud/app-password"
+
+message.send.backend.type = "smtp"
+message.send.backend.host = "smtp.mail.me.com"
+message.send.backend.port = 587
+message.send.backend.encryption.type = "start-tls"
+message.send.backend.login = "you@icloud.com"
+message.send.backend.auth.type = "password"
+message.send.backend.auth.cmd = "pass show icloud/app-password"
+```
+
+**Note:** Generate an app-specific password at appleid.apple.com
+
+## Folder Aliases
+
+Map custom folder names:
+```toml
+[accounts.default.folder.alias]
+inbox = "INBOX"
+sent = "Sent"
+drafts = "Drafts"
+trash = "Trash"
+```
+
+## Multiple Accounts
+
+```toml
+[accounts.personal]
+email = "personal@example.com"
+default = true
+# ... backend config ...
+
+[accounts.work]
+email = "work@company.com"
+# ... backend config ...
+```
+
+Switch accounts with `--account`:
+```bash
+himalaya --account work envelope list
+```
+
+## Notmuch Backend (local mail)
+
+```toml
+[accounts.local]
+email = "user@example.com"
+
+backend.type = "notmuch"
+backend.db-path = "~/.mail/.notmuch"
+```
+
+## OAuth2 Authentication (for providers that support it)
+
+```toml
+backend.auth.type = "oauth2"
+backend.auth.client-id = "your-client-id"
+backend.auth.client-secret.cmd = "pass show oauth/client-secret"
+backend.auth.access-token.cmd = "pass show oauth/access-token"
+backend.auth.refresh-token.cmd = "pass show oauth/refresh-token"
+backend.auth.auth-url = "https://provider.com/oauth/authorize"
+backend.auth.token-url = "https://provider.com/oauth/token"
+```
+
+## Additional Options
+
+### Signature
+```toml
+[accounts.default]
+signature = "Best regards,\nYour Name"
+signature-delim = "-- \n"
+```
+
+### Downloads directory
+```toml
+[accounts.default]
+downloads-dir = "~/Downloads/himalaya"
+```
+
+### Editor for composing
+Set via environment variable:
+```bash
+export EDITOR="vim"
+```
+