Add GitHub Action for automatically formatting code in PRs and branches

.github/workflows/FORMAT_PR.md

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+# Format PR GitHub Action
+
+This GitHub Action allows you to automatically format code in pull requests or branches using the project's `pnpm format` script.
+
+## Usage
+
+### Manual Trigger
+
+1. Go to the [Actions tab](https://github.com/dubzzz/fast-check/actions) in the repository
+2. Click on "Format PR" workflow
+3. Click "Run workflow"
+4. Fill in the required parameters:
+   - **Type of reference**: Choose either "branch" or "pull_request"
+   - **Branch name or PR number**:
+     - For branches: Enter the branch name (e.g., `feature/my-branch`)
+     - For PRs: Enter the PR number (e.g., `123`)
+
+### What it does
+
+1. **Validates input**: Ensures PR numbers are numeric and branch names don't contain spaces
+2. **Checks out the target**: Fetches the specified branch or PR
+3. **Sets up environment**: Installs pnpm and project dependencies
+4. **Checks formatting**: Runs `pnpm format:check` to see if changes are needed
+5. **Applies formatting**: If needed, runs `pnpm format` to fix formatting issues
+6. **Commits changes**: Creates a commit with formatted code and pushes back to the branch/PR
+
+### Example scenarios
+
+- **Format a PR**: Select "pull_request" and enter PR number like `456`
+- **Format a branch**: Select "branch" and enter branch name like `fix-formatting-issues`
+
+### Notes
+
+- The action will only commit changes if formatting modifications are actually needed
+- Commits are made by the `github-actions[bot]` user
+- The action requires write permissions to the repository

.github/workflows/format-pr.yml

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+name: Format PR
+
+on:
+  workflow_dispatch:
+    inputs:
+      ref_type:
+        description: 'Type of reference to format'
+        required: true
+        type: choice
+        options:
+          - branch
+          - pull_request
+      ref_value:
+        description: 'Branch name or PR number'
+        required: true
+        type: string
+
+permissions:
+  contents: write
+  pull-requests: read
+
+jobs:
+  format:
+    name: 'Format code and commit changes'
+    runs-on: ubuntu-latest
+    steps:
+      - name: Determine checkout reference
+        id: ref
+        run: |
+          if [ "${{ github.event.inputs.ref_type }}" = "pull_request" ]; then
+            # Validate PR number is numeric
+            if ! [[ "${{ github.event.inputs.ref_value }}" =~ ^[0-9]+$ ]]; then
+              echo "❌ PR number must be a valid integer"
+              exit 1
+            fi
+            echo "ref=refs/pull/${{ github.event.inputs.ref_value }}/head" >> $GITHUB_OUTPUT
+            echo "target_type=pull_request" >> $GITHUB_OUTPUT
+          else
+            # Validate branch name format (basic validation)
+            if [[ "${{ github.event.inputs.ref_value }}" =~ [[:space:]] ]]; then
+              echo "❌ Branch name cannot contain spaces"
+              exit 1
+            fi
+            echo "ref=${{ github.event.inputs.ref_value }}" >> $GITHUB_OUTPUT
+            echo "target_type=branch" >> $GITHUB_OUTPUT
+          fi
+
+      - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5.0.0
+        with:
+          ref: ${{ steps.ref.outputs.ref }}
+          token: ${{ secrets.GITHUB_TOKEN }}
+          fetch-depth: 0
+
+      - name: Install pnpm
+        uses: pnpm/action-setup@a7487c7e89a18df4991f7f222e4898a00d66ddda # v4.1.0
+
+      - name: Using Node v22.x
+        uses: actions/setup-node@a0853c24544627f65ddf259abe73b1d18a591444 # v5.0.0
+        with:
+          node-version: '22.x'
+          cache: 'pnpm'
+
+      - name: Install dependencies
+        run: pnpm install --frozen-lockfile
+
+      - name: Check current branch info
+        run: |
+          echo "Current branch: $(git branch --show-current)"
+          echo "Current commit: $(git rev-parse HEAD)"
+          echo "Target type: ${{ steps.ref.outputs.target_type }}"
+          echo "Target ref: ${{ github.event.inputs.ref_value }}"
+
+      - name: Check if formatting is needed
+        id: format_check
+        run: |
+          if ! pnpm format:check; then
+            echo "changes_needed=true" >> $GITHUB_OUTPUT
+          else
+            echo "changes_needed=false" >> $GITHUB_OUTPUT
+          fi
+
+      - name: Run format
+        if: steps.format_check.outputs.changes_needed == 'true'
+        run: pnpm format
+
+      - name: Configure git
+        if: steps.format_check.outputs.changes_needed == 'true'
+        run: |
+          git config --local user.email "41898282+github-actions[bot]@users.noreply.github.com"
+          git config --local user.name "github-actions[bot]"
+
+      - name: Commit formatting changes
+        if: steps.format_check.outputs.changes_needed == 'true'
+        run: |
+          git add .
+          if ! git diff --cached --quiet; then
+            echo "Committing formatting changes..."
+            git commit -m "style: auto-format code with prettier
+
+            Applied by GitHub Action workflow: ${{ github.workflow }}
+            Run ID: ${{ github.run_id }}
+            Target: ${{ steps.ref.outputs.target_type }} '${{ github.event.inputs.ref_value }}'"
+
+            echo "Pushing changes..."
+            git push
+          else
+            echo "No changes to commit after formatting"
+          fi
+
+      - name: Output result
+        run: |
+          if [ "${{ steps.format_check.outputs.changes_needed }}" = "true" ]; then
+            echo "✅ Formatting changes have been committed and pushed"
+          else
+            echo "✅ Code is already properly formatted, no changes needed"
+          fi