ci: run PR workflows for stacked PRs

RE
5d ago
+7-8

Description

Right now our PR workflows only trigger on pull_request with branches: [main], so stacked PR (whose base is another PR branch) doesn't run code-quality, integration-test or license checks until its parent merges.

Tested on #578 (throw-away PR stacked on this one).

Checklist

  • Tests pass locally
  • uv run ruff check . passes
  • uv tool run fawltydeps --check-unused --pyenv .venv passes
  • Updated CHANGELOG.md with my changes, if notable

Summary by CodeRabbit

  • New Features
    • Automated code-quality, integration, license, and publishing checks now run on pull requests targeting any branch, not only main. Existing event and path filters remain in place.
  • Documentation
    • CI guidance now covers workflows for pull requests to any branch, including stacked pull requests, and explains how to gate publishing steps.