build: add commit message tooling

Add a repository-local tool to lint, format, and safely reword commit
messages. Preserve markdown lists, quotes, code blocks, and trailers
while enforcing subject and body width limits.

Expose linting, formatting, and rewording through Make targets and
document the supported workflows. Run the linter in CI for pull request
and push commit ranges so malformed messages are caught before merging.

.github/workflows/main.yml

@@ -24,6 +24,37 @@ env:
   LITD_ITEST_BRANCH: master
 
 jobs:
+  ########################
+  # commit message lint
+  ########################
+  commit-message:
+    name: Commit Message
+    runs-on: ubuntu-latest
+    steps:
+      - name: git checkout
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: Lint commit messages
+        env:
+          EVENT_NAME: ${{ github.event_name }}
+          BASE_SHA: ${{ github.event.pull_request.base.sha }}
+          HEAD_SHA: ${{ github.event.pull_request.head.sha }}
+          BEFORE_SHA: ${{ github.event.before }}
+          GITHUB_SHA: ${{ github.sha }}
+        run: |
+          if [ "$EVENT_NAME" = "pull_request" ]; then
+            range="$BASE_SHA..$HEAD_SHA"
+          elif [ "$EVENT_NAME" = "push" ] &&
+               [ "$BEFORE_SHA" != "0000000000000000000000000000000000000000" ]; then
+            range="$BEFORE_SHA..$GITHUB_SHA"
+          else
+            range="$(git rev-parse HEAD^ 2>/dev/null)..HEAD"
+          fi
+
+          make commitmsg-lint range="$range"
+
   ########################
   # RPC compile and check
   ########################