feat(email): add email client tools for agents

[zbigniewsobiecki]

Summary

Implement comprehensive email integration for CASCADE agents, enabling them to interact with email via IMAP (read/search) and SMTP (send/reply).

Core Features

• IMAP client for searching and reading emails using imapflow
• SMTP client for sending emails and replying to threads using nodemailer
• AsyncLocalStorage-based credential scoping via withEmailIntegration()
• Database migration for email integration credentials

Gadgets (SearchEmails, ReadEmail, SendEmail, ReplyToEmail)

• Full schema validation (RFC 2822 subject length, date formats, UID validation)
• Error logging for all gadget operations
• HTML body fallback when text body unavailable
• Proper email threading support (In-Reply-To, References headers)

Agent Execution Integration

• Email credential scoping in manual-runner, GitHub webhook handler, PM webhook handler
• Integration role definitions for email provider credentials
• canAccessEmail capability flag in agent definitions

Code Quality Improvements

• IMAP connection timeouts (30s connect, 15s greeting, 60s socket)
• Awaited transport.close() to prevent connection leaks
• Credential resolution error logging
• Reduced cognitive complexity via helper function extraction
• Fixed pre-existing test failures in file gadget tests

Test plan

• Email integration tests (credential resolution, withEmailIntegration)
• Email client credential scoping tests
• Gadget core function tests (27 tests total)
• All existing tests pass (3313 tests)
• TypeScript type check passes
• Lint check passes (no warnings)

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