Let push policies report ignored events without overwriting the stack’s plan check

Workaround

A GitHub Actions workflow on issue_comment that duplicates the push policy’s approval and mergeability logic and replies on the PR. It can disagree with what Spacelift actually decided, and mergeable_state is computed asynchronously, so it sometimes has to guess.

Problem

We gate applying from a PR on a push policy rule: a `/spacelift deploy <stack>` comment queues a tracked run only when `pull_request.approved` and `mergeable` are true. When it isn’t, the event is ignored and the person who commented gets no feedback. The push policy’s notify and message rules can send the reason back, but only as the stack’s commit status. That uses the same `spacelift/<stack>` check name as the PR’s plan, so the plan’s green result is replaced by “skipped”. Reviewers looking at the checks list then lose the plan result. Notification policies can post PR comments, but they never see ignored push events, so they can’t help either. We’ve had to add a GitHub Actions workflow that re-implements the approval and mergeability checks just to reply on the PR.