[Feat] Make closed PR Slack reaction configurable with default ❌

[roomote-roomote]

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

What changed

Closed PR Slack reactions are no longer hard-coded to heavy_multiplication_x. Deployments now default to :x: (x) and admins can change the emoji under Settings → Vibes → Custom reaction emoji.

Merged PRs still use the existing completion emoji; only closed-without-merge terminal notifications use the new closed-PR preference.

Why this change was made

Teams wanted :x: as the closed-PR signal and a way to tune it the same way acknowledgement and completion reactions already work.

Impact

• Default closed-PR reaction becomes x for new and unset deployments
• Vibes settings gains a required Closed PR emoji field with reset-to-default
• Public Vibes docs include the new setting
• Requires a DB migration for deployment_settings.slack_pr_closed_emoji

Screenshots

[roomote-roomote]

No new code issues found. See task

_{Reviewed b2d6bf6}

[mrubens]

@roomote-roomote resolve conflicts

[roomote-roomote]

I'm on it. I routed this request into the existing PR task so follow-up work stays on one Roomote thread for this PR, and I'll keep updates here. See task

[roomote-roomote]

Resolved merge conflicts in:

• packages/db/drizzle/meta/0011_snapshot.json
• packages/db/drizzle/meta/_journal.json
• packages/types/src/slack.ts

Renumbered this PR's migration to 0012_broad_pretty_boy so it can sit after develop's 0011_generalized_llm_usage. Kept both DEFAULT_SLACK_PR_CLOSED_EMOJI and develop's DEFAULT_SLACK_CANCEL_EMOJI.