docs(notifications): TODO.md claims Discord/Teams/Telegram/Google Chat providers exist — they don't

Summary

packages/notifications/TODO.md checks off Discord, Microsoft Teams, Telegram, and Google Chat as [x] done:

- [x] **Slack** - Webhook support
- [x] **Discord** - Webhook support
- [x] **Email** - Injected send function
- [x] **Webhook** - Generic HTTP webhook
- [x] **Microsoft Teams** - Adaptive Cards via webhook
- [x] **Telegram** - Bot API (sendMessage)
- [x] **Google Chat** - Cards via webhook

But packages/notifications/src/providers/ only contains slack.ts, email.ts, webhook.ts, and base.ts — no matching files for the other four. NotificationChannel (packages/notifications/src/types.ts:1) is typed "slack" | "email" | "webhook" only, and NotificationClient (packages/notifications/src/client.ts) only wires those three up. The alarms-system section of the same TODO.md (line 13) still lists discord/teams/telegram/google-chat as channels an alarm should be able to target, so this looks like real missing functionality rather than just stale docs.

Proposal

Add a Discord provider (webhook-based, mirroring the existing SlackProvider) as a first, self-contained slice:

• New providers/discord.ts following the SlackProvider/WebhookProvider pattern
• Extend NotificationChannel and wire it into NotificationClient
• Tests mirroring the existing Slack provider tests
• Fix the TODO.md checkboxes to reflect reality

Keeping this scoped to Discord only — happy to do Teams/Telegram/Google Chat as separate follow-up PRs if this shape is right, rather than one big PR.

Checking scope/interest before opening a PR, per the AI policy here — let me know if this is wanted or if there's context I'm missing (e.g. planned elsewhere / superseded by a different design).

Disclosure: I used Claude Code to grep the providers directory against TODO.md and the type definitions; verified manually before writing this up.

Start with packages/notifications/src/providers/slack.ts and webhook.ts, then inspect packages/notifications/src/types.ts, client.ts, and the existing Slack provider tests. Add the Discord provider and client/type wiring in the same pattern, update packages/notifications/TODO.md to match the implemented scope, and verify the Discord-focused tests pass.