🏗️✨：validate commit messages

.github/PULL_REQUEST_TEMPLATE.md

@@ -34,7 +34,15 @@ https://github.com/OpenINF/.github/blob/HEAD/CONTRIBUTING.md.
 
 ## Emojis for categorizing pull requests
 
-_Copy and paste one of the following emoji into description_
+The title becomes the subject of the commit that lands, so it has to hold
+to the same rules. They are written up, with the reasoning, at
+https://open.inf.is/docs/handbook/style/commit-messages/ -- in short: a
+category emoji, then optionally an action, then `：` (U+FF1A), then what
+the change does, in 50 characters or fewer and with no `#NNNN` on the end.
+
+_Copy and paste one of the following emoji into description_ -- copy
+rather than type, since some have a lookalike spelling that is not the
+one recognized here.
 
 🏷️ meta
 🐋 dev container

.github/workflows/lint-and-test.yml

@@ -10,6 +10,10 @@ jobs:
     steps:
       - name: Check out project repository
         uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+        with:
+          # `verify.commits` compares against the base branch, which a
+          # single-commit checkout does not contain.
+          fetch-depth: 0
       - name: Set up Node.js runtime
         uses: actions/setup-node@820762786026740c76f36085b0efc47a31fe5020 # v7.0.0
         with:
@@ -63,6 +67,11 @@ jobs:
               - '**/*.yml'
               - '**/*.yaml'
 
+      # Not behind a filter: every pull request has commit messages, whatever
+      # it touches.
+      - name: Verify commit messages
+        run: nps verify.commits
+
       # Use the filter to check if files with a specific file type were changed
       # in the PR. If they were, run the relevant linters. Otherwise, skip.
       - name: Verify Browserslist