Add send, reply, and draft capability

[swinton]

Currently the REPL is read-only (plus batch delete). Adding compose/reply/draft support would make it useful for automation, not just inspection.

Proposed API

// Compose a new message
await send({ to: 'alice@example.com', subject: 'Hello', body: 'Hi there' })

// Reply to an existing message
await reply('1883a5111e8ef47c', 'Thanks!')

// Save a draft
await draft({ to: 'alice@example.com', subject: 'WIP', body: 'Draft body' })

Notes

• users.messages.send
• users.drafts.create
• Messages need to be RFC 2822 formatted and base64url-encoded before sending
• Reply should set In-Reply-To and References headers and attach to the correct threadId
• Requires adding https://www.googleapis.com/auth/gmail.send or https://mail.google.com/ to scopes (the latter is already present)