docs: add a skill for gating CI on GitHub stacks

[raulgg]

Summary

Adds a skill an agent can load while editing GitHub Actions workflows or working a GitHub stack, and an install line under the Quick start agent section.

Why

The paste prompt is a one-shot setup. The skill is for work that is already touching a workflow or a stack. It changes the gate when asked, when a workflow is being created, or when most of one workflow is being rewritten. Smaller edits and stack operations get one question. Other tasks stay quiet.

Changes

• skills/stack-ci-gate/SKILL.md picks Asked, Rewrite, Suggest, or Stop
• A stack counts when the stacks list has an open stack, or a closed stack whose latest pull_requests[].merged_at is within 90 days
• The workflow edit the user asked for still lands. Asking for the gate with no stack gets one sentence and no edit
• A workflow is gated only when a step uses the action and another job or step checks should-run
• Workflow edits stay uncommitted
• README install line: npx skills add raulgg/stack-ci-gate, with -g for every repo

Verification

• npm test
• Unit test for the change (file). No action code changed.
• Live path (opened / stacked / synchronize / remaining-bottom). No skip or run behavior changed.