Track 2: implement direct provider drivers and Nylas exit path

[jjalangtry]

Source plan: docs/b2c-replatform-plan.md (local draft dated 2026-06-10).
Parent: #6

Context

The plan explicitly separates direct provider drivers from the near-term B2C increments. Google, Microsoft, and iCloud should keep working on Nylas until provider-agnostic search/sync are solid. Direct drivers are a later cost/control track and will force per-provider re-auth, so they need their own branch/PR path and rollback plan.

Scope

• Define a provider driver interface that matches the Phase 3 sync unit: list/backfill, delta, fetch thread/message, send, mutate flags/folders, attachment download, disconnect/delete.
• Implement drivers in order: Gmail API, Microsoft Graph, then direct IMAP/SMTP for iCloud using ImapFlow/mailparser.
• Keep Nylas as rollback transport per provider until soak passes.
• Plan and communicate re-auth requirements before flipping any production users.
• Respect provider-specific sync constraints: Gmail history IDs/watch renewal, Graph per-folder delta tokens/lifecycle notifications, iCloud UID cursors and periodic flag reconciliation.

Acceptance

• Driver interface can run side-by-side with Nylas for parity checks.
• Each provider can be cut over independently behind a flag.
• Re-auth path is explicit and tested before any production cutover.
• Nylas dependency can be removed only after all active providers have direct-driver parity and rollback no longer needs it.

External references

• Gmail push/watch renewal: https://developers.google.com/workspace/gmail/api/guides/push
• Gmail sync/history guidance: https://developers.google.com/workspace/gmail/api/guides/sync
• Microsoft Graph message delta: https://learn.microsoft.com/en-us/graph/delta-query-messages
• Apple iCloud IMAP/SMTP settings: https://support.apple.com/en-us/102525