Feature Request: Allow customizing GitHub issue title & body for downtime/uptime events

[leandrogehlen]

Summary

Upptime currently allows customizing commit messages via the commitMessages section in .upptimerc.yml, but the GitHub issue title and body created when a site goes down or comes back up are not configurable. These issue templates are hard-coded inside the action, and there is no user-level way to modify them.

I would like Upptime to support customizable templates for:

Issue titles (e.g., “⚠️ $SITE is DOWN”)

Issue bodies (e.g., include custom formatting, links, instructions, checklists, etc.)

Ideally, this would work similarly to commitMessages, perhaps:

issueTemplates:
  down:
    title: "🔴 $SITE_NAME is down"
    body: |
      We detected that **$SITE_NAME** is unreachable.
      - Status: $STATUS
      - Response code: $RESPONSE_CODE
      - Response time: $RESPONSE_TIME ms
      - Timestamp: $DATETIME

  up:
    title: "🟢 $SITE_NAME is back up"
    body: |
      Good news! **$SITE_NAME** is responding again.
      - Status: $STATUS
      - Response code: $RESPONSE_CODE
      - Response time: $RESPONSE_TIME ms
      - Timestamp: $DATETIME