[Feat] Add Discord manager automation destinations

[roomote-community]

Opened on behalf of Matt Rubens. Follow up by mentioning @roomote, in the web UI, or in Discord.

What changed

• Added Discord channels to the shared manager automation destination picker while preserving Slack channel and manual/private channel support.
• Persisted and validated the Discord manager destination and carried it through automation runtime resolution.
• Preserved saved Discord destinations when older settings clients omit the new manager destination field.
• Applied the shared Discord destination to manager-facing automation output, including platform configuration alerts and new custom automation defaults.

Why this change was made

Manager-facing automations previously required Slack for their shared output destination even though individual automations already supported Discord. Deployments can now use Discord consistently for manager updates.

Impact

Admins can choose either Slack or Discord as the shared manager destination. Existing Slack configurations continue to work, while Discord-only deployments can route manager summaries, suggestions, alerts, and other inherited automation output to a selected Discord channel.

[roomote-community]

No code issues found. See task

• apps/web/src/trpc/commands/automations/settings-update.ts:393-473 — Preserve the saved Discord manager destination when a legacy manager-channel save omits managerDiscordChannel and does not select Slack. The optional field is currently normalized to null and persisted, so a pre-deploy settings tab can erase a newly configured Discord manager channel.

_{Reviewed 1857b12}

[roomote-community]

Fixed the legacy manager-channel save path: omitted Discord input now preserves the stored destination, while explicit clears and Slack selections keep their existing behavior. Added regression coverage and pushed 1857b12. See task.