ci: Remove check workflow and filter on branch name (#253)

Removes the check workflow and using it to deduplicate PRs and pushes:
CI should be run on all PRs and also on `main` branch: pushes to PRs are
already picked up by the pull_request filtering: "By default, a workflow
only runs when a pull_request event's activity type is opened,
synchronize, or reopened.", the definition of `synchronize` can be found
in the documentation: "A pull request's head branch was updated. For
example, the head branch was updated from the base branch or new commits
were pushed to the head branch."

E: TC believes someone at some point wanted CI to run on branches that
do not have open PRs, because when a PR is opened he gets notified
whether it is draft or not. This does require that now a draft PR is
opened for CI to run, but does anyone have a use case for running CI
against a branch without even a draft PR?

Author: DiamondJoseph

.github/workflows/_check.yml

@@ -2,22 +2,19 @@ name: CI
 
 on:
   push:
+    branches:
+      - main
+    tags:
+      - '*'
   pull_request:
 
 jobs:
-  check:
-    uses: ./.github/workflows/_check.yml
-
   lint:
-    needs: check
-    if: needs.check.outputs.branch-pr == ''
     uses: ./.github/workflows/_tox.yml
     with:
       tox: pre-commit
 
   test:
-    needs: check
-    if: needs.check.outputs.branch-pr == ''
     uses: ./.github/workflows/_test.yml
     with:
       python-version: dev
@@ -26,8 +23,6 @@ jobs:
       CODECOV_TOKEN: ${{ secrets.CODECOV_TOKEN }}
 
   docs:
-    needs: check
-    if: needs.check.outputs.branch-pr == ''
     uses: ./.github/workflows/_docs.yml
     permissions:
       contents: write

.github/workflows/ci.yml

@@ -0,0 +1,24 @@
+# 21. Run CI on pull requests and main branch only
+
+Date: 2025-05-06
+
+## Status
+
+Accepted
+
+## Context
+
+CI has been being run on branches that do not have open pull requests, which has: added spurious jobs to the organisation's compute limit; made workflow definitions more complex; made the results page harder to parse; and allowed known working development branches to become stale or forgotten.
+
+Running CI only on branches that have open pull requests will: reduce compute jobs created for code that is known to not be working; simplify workflow definitions; simplify the CI results page; encourage early creation of pull requests, adding visibility to development.
+
+Draft pull requests will still have CI jobs run, ensuring that code quality is maintained while allowing reviewers to ignore changes that are not ready for human review.
+
+## Decision
+
+CI will only be run on all PRs when changes are made, and on the main branch when PRs are merged into it: not on branches that do not have open PRs.
+Opening a draft PR will allow CI to be run, with the understanding that the PR may not be ready for review.
+
+## Consequences
+
+The `check` job will be removed and CI will run on all PRs and on the `main` branch.

docs/explanations/decisions/0021-ci-only-pr.md

@@ -5,19 +5,13 @@ on:
   pull_request:
 
 jobs:
-  check:
-    uses: ./.github/workflows/_check.yml
 
   lint:
-    needs: check
-    if: needs.check.outputs.branch-pr == ''
     uses: ./.github/workflows/_tox.yml
     with:
       tox: pre-commit,type-checking
 {% raw %}
   test:
-    needs: check
-    if: needs.check.outputs.branch-pr == ''
     strategy:
       matrix:
         runs-on: ["ubuntu-latest"] # can add windows-latest, macos-latest
@@ -35,21 +29,15 @@ jobs:
       CODECOV_TOKEN: ${{ secrets.CODECOV_TOKEN }}
 {% endraw %}{% if docker %}
   container:
-    needs: check
-    if: needs.check.outputs.branch-pr == ''
     uses: ./.github/workflows/_container.yml
     permissions:
       contents: read
       packages: write
 {% endif %}{% if sphinx %}
   docs:
-    needs: check
-    if: needs.check.outputs.branch-pr == ''
     uses: ./.github/workflows/_docs.yml
 {% endif %}
   dist:
-    needs: check
-    if: needs.check.outputs.branch-pr == ''
     uses: ./.github/workflows/_dist.yml
   {% if pypi %}
   pypi: