ci: use lockfile-pinned commitlint instead of floating runtime install

The runtime 'yarn add commitlint@20.5.3' pulled @commitlint/cli v21,
which requires Node >=22.12.0 while the job runs Node 20.x, breaking
the commitlint check on every branch. Use the @commitlint/cli ^19.5.0
already installed via --frozen-lockfile instead.

Author: sallymoc

.github/workflows/commitlint.yml

@@ -29,11 +29,6 @@ jobs:
       - name: Install dependencies
         run: yarn install --frozen-lockfile
 
-      - name: Install commitlint
-        run: |
-          yarn add conventional-changelog-conventionalcommits
-          yarn add commitlint@20.5.3
-
       - name: Validate current commit (last commit) with commitlint
         if: github.event_name == 'push'
         run: npx commitlint --last --verbose