Complete channel provider foundation for Gmail and IMAP/SMTP

[zaridan]

Make the first channel layer credible without making Helpthread email-only.

Current state:

• Gmail OAuth connect is implemented at the API/service layer.
• Gmail push and bounded Gmail reconciliation exist.
• Provider interfaces exist for inbound email and outbound send.
• IMAP/SMTP is not implemented yet; the draft spec currently identifies unresolved correctness blockers.

Acceptance:

• Channel/provider interfaces clearly separate core conversation primitives from email-specific behavior.
• Gmail OAuth, push, reconciliation, send, disconnect, and reconnect behavior are tested and documented.
• The mailbox connection UI uses the public API rather than private implementation paths.
• IMAP/SMTP work only moves forward once scheduled-fetch, idempotency, self-echo suppression, and cursor semantics are made buildable.
• Future non-email channels have an obvious place to attach without rewriting the conversation model.