Tasks

• Gmail messages/drafts/threads get and history list: hand-written output types replacing the Message wrapper, including GmailMessageHeaderOutput for the fetched headers.
• Gmail settings readers: transparent newtypes over the io-gmail resource for delegates, forwarding addresses, filters and send-as; hand-written for the vacation, imap, pop, language and auto-forwarding singletons, whose io-gmail types lack JsonSchema.
• Microsoft Graph message get: transparent newtype over MsgraphMessage, matching what message list already emits.
• Drop the fake HistoryOutput schema type that documented the string wrapper.
• messages get and threads get: narrow headers client-side through a shared message_headers, case-insensitive, order and repeats preserved.
• drafts get --format raw: decode and write the RFC 5322 bytes, like messages get --format raw.
• Register all fourteen new schemas in src/json_schema.rs; always emit Vec fields so the schema matches the payload.
• Tests for the header filter; changelog.
• Build/test/fmt/clippy.
• Fold into cairn/spec/commands.md; log; land.