refactor(email): registry-pattern provider architecture + code-review fixes

[zbigniewsobiecki]

Summary

Replaces the monolithic EmailClient class with a clean, extensible registry-based architecture modelled after the existing PM integration pattern.

Architecture changes

• EmailProvider interface — runtime operations (searchEmails / readEmail / sendEmail / replyToEmail / markEmailAsSeen)
• EmailIntegration interface — DB credential resolution + AsyncLocalStorage provider scoping per project
• EmailIntegrationRegistry — singleton populated at import time via src/email/index.ts (same pattern as pm/index.ts)
• ImapEmailProvider — password-auth via imapflow (IMAP) + nodemailer (SMTP)
• GmailEmailProvider — OAuth-auth via imapflow (IMAP) + Gmail REST API (send) — avoids SMTP 465 blocking in containers
• ImapIntegration / GmailIntegration — resolve credentials from DB, scope provider via withEmailProvider

Adding a new email provider now requires only: a new EmailProvider class + one emailRegistry.register() call.

Code-review fixes (gadget core + integration)

File	Fix
All 5 src/gadgets/email/core/*.ts	Hoist const message before logger.error — was extracted twice
readEmail.ts	Add else { lines.push('', '(no body)') } fallback for attachment-only emails
sendEmail.ts + replyToEmail.ts	Guard empty accepted list — was emitting "Email sent successfully to "
searchEmails.ts	Remove embedded \n from header line — caused double blank line in output
imap/integration.ts	logger.warn when IMAP/SMTP port is non-numeric — was silently returning null

Test coverage

• context.test.ts — AsyncLocalStorage scoping, getEmailProvider throws outside scope
• imap/adapter.test.ts — adds readEmail (success + not-found) and replyToEmail (threading headers, finally cleanup)
• gmail/adapter.test.ts — adds searchEmails + readEmail tests; migrates mocks to vi.hoisted() for correctness
• integration.test.ts — registry delegation, credential fallback, warn path

Test plan

• npm test — 3430 tests pass
• npm run typecheck — zero errors
• npm run lint — zero warnings

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