Add commit message check

.github/workflows/ci.yml

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+name: ci
+
+on:
+  pull_request:
+  merge_group:
+  push:
+    branches:
+      - main
+
+permissions:
+  contents: read
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v7.0.0
+        with:
+          fetch-depth: 0
+          persist-credentials: false
+      - name: Test commit message checker
+        run: python3 check-commit-messages/test_check.py
+      - name: Check commit messages
+        if: github.event_name == 'pull_request'
+        uses: ./check-commit-messages
+        with:
+          base_ref: ${{ github.event.pull_request.base.ref }}

.gitignore

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+__pycache__/
+*.pyc

README.md

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+# GitHub Actions
+
+Shared GitHub Actions for Airlift projects.
+
+## Available actions
+
+### Check commit messages
+
+Checks pull request commit titles, description wrapping, and attribution
+trailers. See [check-commit-messages](check-commit-messages/README.md) for
+usage and policy details.

check-commit-messages/README.md

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+# Check commit messages
+
+Checks every non-merge commit in a pull request.
+
+## Policy
+
+- Commit titles should be at most 50 characters and must not exceed 60.
+- Commit descriptions should wrap at 72 characters. Ordinary text must not
+  exceed 79 characters.
+- `Assisted-by` and `Co-authored-by` trailers must not credit common AI models
+  or coding tools. Human attribution remains allowed.
+
+Long URLs, recognized trailers, quoted text, fenced code blocks, and long
+unwrappable tokens are exempt from description wrapping.
+
+## Usage
+
+The action requires a checkout with complete history:
+
+```yaml
+check-commit-messages:
+  if: github.event_name == 'pull_request'
+  runs-on: ubuntu-latest
+  steps:
+    - uses: actions/checkout@v7.0.0
+      with:
+        fetch-depth: 0
+        persist-credentials: false
+    - uses: airlift/github-actions/check-commit-messages@<commit-sha> # v1
+      with:
+        base_ref: ${{ github.event.pull_request.base.ref }}
+```
+
+Consumers should pin the action to a complete commit SHA.

check-commit-messages/action.yml

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+name: Check commit messages
+description: Check commit title length, description wrapping, and attribution trailers
+
+inputs:
+  base_ref:
+    description: Pull request base branch
+    required: true
+
+runs:
+  using: composite
+  steps:
+    - name: Check commit messages
+      shell: bash
+      env:
+        BASE_REF: ${{ inputs.base_ref }}
+      run: |
+        if [[ ! "$BASE_REF" =~ ^[A-Za-z0-9._/-]+$ ]]; then
+          echo "Invalid pull request base ref: $BASE_REF" >&2
+          exit 1
+        fi
+        python3 "${{ github.action_path }}/check.py" \
+          "origin/${BASE_REF}..HEAD"