ci: replace third-party action with inline conventional-commit regex

The Stainless TS/Python repos restrict GitHub Actions to an allowlist
that doesn't include amannn/action-semantic-pull-request. Using a
pure-shell regex check keeps the workflow self-contained and removes
the third-party dependency, which also reduces supply-chain risk.

Author: declan-scale

.github/workflows/lint-pr.yaml

@@ -14,30 +14,35 @@ jobs:
   validate-pr-title:
     name: Validate PR title (Conventional Commits)
     runs-on: ubuntu-latest
-    permissions:
-      statuses: write
-      pull-requests: read
+    if: ${{ !contains(github.event.pull_request.labels.*.name, 'skip-conventional-commit-check') }}
     steps:
-      - uses: amannn/action-semantic-pull-request@48f256284bd46cdaab1048c3721360e808335d50 # v6.1.1
+      - name: Check Conventional Commits format
         env:
-          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
-        with:
-          types: |
-            feat
-            fix
-            docs
-            style
-            refactor
-            test
-            chore
-            ci
-            build
-            perf
-            revert
-          requireScope: false
-          # Allow skipping this check with a label (e.g. for automated/bot PRs)
-          ignoreLabels: |
-            skip-conventional-commit-check
+          PR_TITLE: ${{ github.event.pull_request.title }}
+        run: |
+          # Conventional Commits: <type>(<optional-scope>)(!): <subject>
+          # Allowed types match the project's release-please changelog sections.
+          PATTERN='^(feat|fix|docs|style|refactor|test|chore|ci|build|perf|revert)(\([^)]+\))?!?: .+'
+
+          if printf '%s' "$PR_TITLE" | grep -qE "$PATTERN"; then
+            echo "PR title is a valid Conventional Commit: $PR_TITLE"
+            exit 0
+          fi
+
+          {
+            echo "::error title=Invalid PR title::PR title must follow Conventional Commits format."
+            echo "  Got:      $PR_TITLE"
+            echo "  Expected: <type>(<optional-scope>)(!): <subject>"
+            echo "  Types:    feat, fix, docs, style, refactor, test, chore, ci, build, perf, revert"
+            echo ""
+            echo "  Examples:"
+            echo "    feat: add new endpoint"
+            echo "    fix(client): handle empty response"
+            echo "    chore!: drop python 3.11 support"
+            echo ""
+            echo "  Add the 'skip-conventional-commit-check' label to bypass this check."
+          } >&2
+          exit 1
 
   validate-pr-base:
     name: Validate PR base branch