[Fix] Merge announcer destinations reset after saving

[roomote-roomote]

​Opened on behalf of @mrubens. Follow up by mentioning @roomote-roomote, in the web UI, or in Slack.

What changed

• Preserve Merge announcer provider, Channel/DM mode, and destination ID at the automations.updateSettings tRPC boundary so non-default destinations reach persistence and rehydrate after reload.
• Add a database-backed regression test that parses the router-owned destination schema, saves a non-default channel, reloads deployment settings, and verifies the same provider, mode, and channel.
• Give Merge announcer a distinct commit-activity icon while keeping the merged-PR digest's megaphone, including the generated Slack card asset.
• Replace raw Slack Markdown with the standard Roomote automation card: icon/title header, conversational push context, blockquoted summary, and View changes / Configure actions.
• Give Teams, Telegram, and Discord the equivalent concise narrative, detail quote, and action links through their existing communication adapters.

Why this change was made

The destination picker added in PR #1845 built the correct save payload, but the inline tRPC Zod schema omitted its three destination fields and stripped them before the mutation ran. The same release also reused the merged-PR icon and posted standard Markdown directly into Slack's mrkdwn surface, producing ambiguous icons and literal formatting.

Impact

Admins can save any supported Merge announcer channel or DM destination and see it remain selected after reload. Push notifications now follow Roomote's standard automation-card hierarchy, avoid repetitive author dumps and raw Markdown, and visually distinguish immediate primary-branch activity from the recurring merged-PR digest.

Screenshots

[roomote-community]

No code issues found. See task

_{Reviewed 1b10dfd}