#22185: Add Email channel MVP plugin (SMTP outbound)

## Summary Implements issue #22183 with an Email channel MVP. ### What was added - New plugin: `extensions/email` - New channel id: `email` (plugin metadata + docs integration) - Outbound SMTP delivery via `nodemailer` - Target normalization for: - `user@example.com` - `email:user@example.com` - `mailto:user@example.com` - Channel status probe using SMTP `verify` - New docs page: `/channels/email` - Channels index entry for Email ### Config (MVP) `channels.email` supports: - `enabled` - `smtpHost`, `smtpPort`, `smtpSecure` - `smtpUser`, `smtpPass`, `smtpPassEnv` - `from` - `subjectPrefix` - `dmPolicy`, `allowFrom` (for policy compatibility) ## Validation - `corepack pnpm lint` - `corepack pnpm exec vitest run extensions/email/src/send.test.ts` - `corepack pnpm test:fast` ## Scope note This PR is intentionally MVP-scoped to outbound SMTP. Inbound email ingestion (IMAP/webhook) is not included yet. Closes #22183