ci: run pre-commit on changed files for PRs, drop fail_fast, document local setup

.github/workflows/master.yml

@@ -24,6 +24,10 @@ jobs:
     steps:
       - name: Checkout
         uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5.0.0
+        with:
+          # Full history is required so `--from-ref`/`--to-ref` below can diff
+          # the PR's base and head commits.
+          fetch-depth: 0
 
       - name: Setup python
         uses: actions/setup-python@a26af69be951a213d495a4c3e4e4022e16d87065 # v5.6.0
@@ -41,3 +45,11 @@ jobs:
 
       - name: Run pre-commit
         uses: pre-commit/action@2c7b3805fd2a0fd8c1884dcaebf91fc102a13ecd # v3.0.1
+        with:
+          # Pull requests: only check the files the PR actually changes, so lint
+          # debt elsewhere on master cannot block an unrelated contribution.
+          # Pushes to master and manual runs: check everything.
+          extra_args: >-
+            ${{ github.event_name == 'pull_request'
+                && format('--from-ref {0} --to-ref {1}', github.event.pull_request.base.sha, github.event.pull_request.head.sha)
+                || '--all-files' }}

.pre-commit-config.yaml

@@ -1,10 +1,12 @@
 ---
-# pre-commit install --allow-missing-config &&  pre-commit autoupdate && pre-commit run --all-files --color auto
+# Local setup: pip install pre-commit && pre-commit install
+# Run everything:  pre-commit run --all-files
 exclude: |
   (?x)^(
     old-versions/.*
   )
-fail_fast: true
+# Report every failing hook in one run instead of stopping at the first one.
+fail_fast: false
 default_stages:
   - pre-commit
 default_language_version:

README.md

@@ -143,6 +143,18 @@ However, when using CI/CD tools to provide automation, keep in mind that the too
 
 Contributions are welcome: fix a typo, add a tool, or propose a new topic by opening an issue or pull request. Please keep tool lists vendor-neutral and alphabetically ordered, and update the Table of Contents when adding or renaming files (see [doc-utilities](doc-utilities/README.md) for the TOC generator).
 
+### Running the checks locally
+
+CI runs [pre-commit](https://pre-commit.com/) (Markdown lint, trailing whitespace, end-of-file newline, YAML/JSON validity) on every pull request. Running the same hooks locally before you push avoids a red CI run:
+
+```bash
+python3.13 -m pip install pre-commit
+pre-commit install          # run the hooks automatically on every commit
+pre-commit run --all-files  # or run them once across the whole repository
+```
+
+On pull requests CI only checks the files you changed; pushes to `master` check the whole repository.
+
 ## Previous versions
 
 Earlier editions are kept in [old-versions](old-versions/) (V0.1, V0.2, V0.3).