Add multi-account Gmail polling with per-account seen tracking, updated
onboarding flow, and config/env resolution.
Based on jasoncarreira's work in #214, rebased onto current main and
cleaned up:
- parseGmailAccounts() extracted to polling/service.ts with 10 unit tests
- Per-account seen email tracking (Map<string, Set<string>>) with legacy
migration from single-account format
- Onboarding supports multi-select for existing accounts + add new
- Config resolution: polling.gmail.accounts > integrations.google.accounts
(legacy) > GMAIL_ACCOUNT env (comma-separated)
- GoogleAccountConfig type for per-account service selection
- Updated docs/configuration.md
Closes#214.
Written by Cameron ◯ Letta Code
"Good artists copy, great artists steal." - Pablo Picasso
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