Support GitHub reactions as approval candidates

[rstuhlmuller]

Feature request

Please consider supporting GitHub reaction objects as approval candidates or policy predicates.

Use case

Some GitHub Apps and review bots signal a successful review by adding a +1
reaction to the pull request issue instead of posting a pull request comment or
submitting an approving review. In that case, Policy Bot cannot currently model
the bot signal directly with policy configuration.

For example, a policy author may want a rule like:

approval_rules:
  - name: review bot reacted
    options:
      methods:
        reactions:
          issue:
            content: "+1"
    requires:
      count: 1
      users:
        - "chatgpt-codex-connector[bot]"

The exact shape is only a strawman. The important part is being able to count a
GitHub reaction from a required user/app account the same way Policy Bot can
currently count comments, comment patterns, GitHub reviews, review-comment
patterns, and pull-request body patterns.

Current workaround

The workaround is to run a separate bridge check that calls GitHub's reactions
API, verifies that the required bot account added a +1 reaction to the PR
issue, then publishes a commit status or check run that Policy Bot can require
through has_status.

That works, but it means every repository or organization that wants to use this
signal needs to maintain extra automation just to translate a GitHub approval
signal into something Policy Bot already understands.

Semantics to consider

• The first useful target would be reactions on the pull request issue itself:
  GET /repos/{owner}/{repo}/issues/{issue_number}/reactions?content=%2B1.
• It would be helpful if reaction approval candidates honored existing
  requires.users, requires.organizations, and requires.teams checks.
• If invalidate_on_push is enabled, a reaction created before the latest
  relevant push should probably be treated similarly to a stale approval
  comment, using the reaction created_at timestamp.
• If GitHub does not send webhook events for reaction changes, it would still be
  useful for Policy Bot to read reactions during normal policy evaluation and
  document the re-evaluation behavior.

Why this belongs in Policy Bot

Policy Bot already lets teams model nuanced approval signals declaratively. A
reaction approval method would keep the policy in one place and avoid requiring
custom status-bridge automation for bots whose only public approval signal is a
GitHub reaction.

[asvoboda]

I don't think Github supports sending webhooks on reactions (https://github.com/orgs/community/discussions/20824 seems to be about issue comments, but I think it probably generalizes to other reaction types). I also can't find anything obvious in https://docs.github.com/en/webhooks/webhook-events-and-payloads. Do you know if Github supports webhooks for reactions? If not, I think the lack of webhooks would effectively make policy-bot appear broken since it can't respond to user interactions for reactions.