feat: rework PR bump preview workflow

Replace the previous draft (poetry-based) attempt with a simpler,
robust implementation:

* Use commitizen-tools/setup-cz instead of installing Poetry/cz from
  the project lockfile. The action repo doesn't need its own Python
  toolchain to run cz.
* Trigger on pull_request_target so the workflow has pull-requests:
  write even for fork PRs. Only `cz bump --dry-run` is executed, so
  PR-controlled scripts are not run.
* Capture the dry-run exit status. Treat exit code 21
  (NoneIncrementExit) as 'no eligible commits' instead of a hard
  failure, and surface other non-zero codes in the comment.
* Use a hidden HTML marker so the comment is replaced (rather than
  duplicated) on every push.

Closes commitizen-tools/commitizen#1510

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: bearomorphism

.github/workflows/pr-bump-preview.yml

@@ -0,0 +1,82 @@
+name: PR bump preview
+
+on:
+  pull_request_target:
+    types: [opened, reopened, synchronize, ready_for_review]
+
+permissions:
+  contents: read
+  pull-requests: write
+
+jobs:
+  bump-preview:
+    if: ${{ github.event.pull_request.draft == false }}
+    runs-on: ubuntu-latest
+    steps:
+      # Using pull_request_target so the workflow has pull-requests: write
+      # access for fork PRs. The job only runs `cz bump --dry-run`, a
+      # read-only command, so PR-controlled scripts are not executed.
+      - name: Check out PR head
+        uses: actions/checkout@v4
+        with:
+          ref: ${{ github.event.pull_request.head.sha }}
+          fetch-depth: 0
+          fetch-tags: true
+
+      - name: Set up Commitizen
+        uses: commitizen-tools/setup-cz@main
+        with:
+          set-git-config: false
+
+      - name: Run cz bump --dry-run
+        id: dry-run
+        run: |
+          set +e
+          output="$(cz bump --dry-run --yes 2>&1)"
+          status=$?
+          set -e
+          {
+            echo "status=${status}"
+            echo "output<<__CZ_BUMP_PREVIEW__"
+            printf '%s\n' "${output}"
+            echo "__CZ_BUMP_PREVIEW__"
+          } >> "$GITHUB_OUTPUT"
+
+      - name: Build comment body
+        env:
+          STATUS: ${{ steps.dry-run.outputs.status }}
+          OUTPUT: ${{ steps.dry-run.outputs.output }}
+        run: |
+          {
+            echo "<!-- commitizen-bump-preview -->"
+            echo "## 🔍 Commitizen bump preview"
+            echo ""
+            case "${STATUS}" in
+              0)
+                echo "Merging this PR will produce the following bump:"
+                echo ""
+                echo '```'
+                printf '%s\n' "${OUTPUT}"
+                echo '```'
+                ;;
+              21)
+                echo "No commits in this PR are eligible for a version bump."
+                ;;
+              *)
+                echo "⚠️ \`cz bump --dry-run\` exited with status \`${STATUS}\`:"
+                echo ""
+                echo '```'
+                printf '%s\n' "${OUTPUT}"
+                echo '```'
+                ;;
+            esac
+          } > comment.md
+
+      - name: Post or update PR comment
+        uses: peter-evans/create-or-update-comment@v4
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}
+          issue-number: ${{ github.event.pull_request.number }}
+          body-path: comment.md
+          body-includes: "<!-- commitizen-bump-preview -->"
+          edit-mode: replace