feat(gmail): add replyToMessage, forwardMessage, and complete human-parity email operations

[AojdevStudio]

Summary

The Gmail service should support all standard email operations a human would perform. Currently missing: direct reply (threaded), forward, reply-all, add/download attachments, and trash/delete.

Problem

Right now sendMessage and createDraft accept inReplyTo but threading is unreliable (especially with mailing list / ConvertKit emails). There's no explicit reply operation that guarantees correct threading with In-Reply-To + References headers. Forwarding is completely absent.

Proposed Operations

Priority 1 — Core

Operation	Purpose	Gmail API
replyToMessage	Reply to a specific message (auto-sets subject, threading headers, recipient)	users.messages.send with proper In-Reply-To, References, threadId
replyAllToMessage	Reply-all (includes CC recipients from original)	Same as above, auto-populates all recipients
forwardMessage	Forward a message to new recipients with original content quoted	users.messages.send with forwarded body + attachments

Priority 2 — Attachment Handling

Operation	Purpose	Gmail API
listAttachments	List attachments on a message (name, size, mimeType)	users.messages.attachments.get
downloadAttachment	Download an attachment by ID	users.messages.attachments.get
sendWithAttachments	Send/draft with file attachments	Multipart MIME message

Priority 3 — Message Management

Operation	Purpose	Gmail API
trashMessage	Move message to trash	users.messages.trash
untrashMessage	Restore from trash	users.messages.untrash
deleteMessage	Permanently delete (skip trash)	users.messages.delete
markAsRead	Remove UNREAD label	users.messages.modify (already possible via modifyLabels but a convenience alias)
archiveMessage	Remove INBOX label	users.messages.modify (same — convenience alias)

Threading Fix for replyToMessage

The key issue with current inReplyTo is that we pass the Gmail message ID instead of building proper MIME headers. A correct reply needs:

1. Set threadId to the original message's threadId
2. Set In-Reply-To header to the original message's Message-ID header value
3. Set References header to original's References + Message-ID
4. Prefix subject with Re: if not already present
5. Auto-populate To from the original sender

replyToMessage should handle all of this from just a messageId input.

Human-Parity Checklist

After this + #42 (draft management), Gmail operations should cover:

• Read messages (getMessage, searchMessages, listMessages)
• Read threads (getThread, listThreads)
• Send new email (sendMessage)
• Create draft (createDraft)
• Send draft (sendDraft)
• Label management (modifyLabels, listLabels)
• Reply to email ← this issue
• Reply-all ← this issue
• Forward email ← this issue
• List/get/update/delete drafts ← feat(gmail): add listDrafts, getDraft, updateDraft, and deleteDraft operations #42
• Attachments (list/download/send) ← this issue
• Trash/delete messages ← this issue

Acceptance Criteria

• replyToMessage({ messageId, body }) sends a correctly threaded reply
• replyAllToMessage({ messageId, body }) includes all original recipients
• forwardMessage({ messageId, to, body? }) forwards with quoted original
• Threading works reliably with mailing lists and marketing platforms (ConvertKit, Kit, etc.)
• Attachment operations functional for common file types
• Trash/delete operations with appropriate safety checks

[linear]

GDRIVE-12