[enhancement] E5-5: Missing notification channels — no Slack, email, or PagerDuty

[OneStepAt4time]

Finding

ID: Enterprise Gap | Severity: 🟠 HIGH

Only Telegram and generic webhook exist. No Slack, email, PagerDuty, or Microsoft Teams integration.

Fix Required

Implement at minimum:

• SlackChannel: Incoming Webhooks or Slack API; session events → channel messages
• EmailChannel: SMTP via Nodemailer; stall/dead events → ops email

Files: New src/channels/slack.ts, src/channels/email.ts

Acceptance Criteria

A stalled session triggers a Slack notification and an email.

Milestone

M-E5: API & Integration

[OneStepAt4time]

Implemented in PR #1530 (merged). SlackChannel + EmailChannel added to src/channels/. Documentation in docs/integrations/notifications.md.