fix: check actual PR commits instead of merge commit HEAD

When actions/checkout runs on a pull_request event, it checks out an
auto-generated merge commit at refs/pull/{N}/merge as HEAD. The commit
message of this merge commit is 'Merge {sha} into {base}', which:
- Matches the (Merge).* alternative in the conventional commits regex
- Is allowed by allow_merge_commits = true

This caused the action to always pass on PRs regardless of the actual
commit messages, since only the merge commit HEAD was ever checked.

Fix by adding get_pr_commit_messages() which, when GITHUB_BASE_REF is
set (indicating a PR context), retrieves the real PR commits via:

  git log --no-merges origin/{base_ref}..HEAD --format=%B%x00 --reverse

Each commit message is then piped individually to commit-check --message
via stdin so the actual PR commits are validated.

In non-PR contexts (push, manual dispatch, etc.) the existing behaviour
is preserved: commit-check determines what to check from git itself.

Also fixes stderr being silently discarded (subprocess.PIPE) by using
subprocess.STDOUT so errors are captured in result.txt and surfaced in
the job summary and PR comments.

Fixes #184

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

Author: shenxianpeng

main.py

@@ -36,35 +36,94 @@ def log_env_vars():
     print(f"PR_COMMENTS = {PR_COMMENTS}\n")
 
 
-def run_commit_check() -> int:
-    """Runs the commit-check command and logs the result."""
-    args = [
-        "--message",
-        "--branch",
-        "--author-name",
-        "--author-email",
-    ]
-    args = [
-        arg
-        for arg, value in zip(
-            args,
+def get_pr_commit_messages() -> list[str] | None:
+    """Get commit messages for all commits in a PR, excluding merge commits.
+
+    In a GitHub Actions PR context, HEAD points to an auto-generated merge commit
+    (refs/pull/{N}/merge), not the actual PR commits. This function retrieves the
+    real commit messages so they can be validated individually.
+
+    Returns None if not in a PR context or if no commits are found.
+    """
+    base_ref = os.getenv("GITHUB_BASE_REF", "")
+    if not base_ref:
+        return None
+
+    try:
+        result = subprocess.run(
             [
-                MESSAGE,
-                BRANCH,
-                AUTHOR_NAME,
-                AUTHOR_EMAIL,
+                "git",
+                "log",
+                "--no-merges",
+                f"origin/{base_ref}..HEAD",
+                "--format=%B%x00",
+                "--reverse",
             ],
+            stdout=subprocess.PIPE,
+            stderr=subprocess.PIPE,
+            text=True,
+            check=False,
         )
-        if value == "true"
+        if result.returncode != 0 or not result.stdout.strip():
+            return None
+        messages = [m.strip() for m in result.stdout.split("\x00") if m.strip()]
+        return messages if messages else None
+    except Exception:
+        return None
+
+
+def run_commit_check() -> int:
+    """Runs the commit-check command and logs the result."""
+    other_check_flags = [
+        ("--branch", BRANCH),
+        ("--author-name", AUTHOR_NAME),
+        ("--author-email", AUTHOR_EMAIL),
     ]
+    other_args = [arg for arg, value in other_check_flags if value == "true"]
 
-    command = ["commit-check"] + args
-    print(" ".join(command))
+    ret_code = 0
     with open("result.txt", "w") as result_file:
-        result = subprocess.run(
-            command, stdout=result_file, stderr=subprocess.PIPE, check=False
-        )
-        return result.returncode
+        if MESSAGE == "true":
+            commit_messages = get_pr_commit_messages()
+            if commit_messages:
+                # PR context: check each commit message individually to avoid
+                # only checking the auto-generated merge commit at HEAD.
+                for msg in commit_messages:
+                    command = ["commit-check", "--message"]
+                    print(" ".join(command))
+                    result = subprocess.run(
+                        command,
+                        input=msg,
+                        text=True,
+                        stdout=result_file,
+                        stderr=subprocess.STDOUT,
+                        check=False,
+                    )
+                    ret_code += result.returncode
+            else:
+                # Non-PR context: let commit-check determine what to check from git.
+                command = ["commit-check", "--message"]
+                print(" ".join(command))
+                result = subprocess.run(
+                    command,
+                    stdout=result_file,
+                    stderr=subprocess.STDOUT,
+                    check=False,
+                )
+                ret_code += result.returncode
+
+        if other_args:
+            command = ["commit-check"] + other_args
+            print(" ".join(command))
+            result = subprocess.run(
+                command,
+                stdout=result_file,
+                stderr=subprocess.STDOUT,
+                check=False,
+            )
+            ret_code += result.returncode
+
+    return ret_code
 
 
 def read_result_file() -> str | None: