option: auto-detect default base branch from remote

Instead of hardcoding 'main' as the default base branch, use
git symbolic-ref to detect the remote's default branch. This
makes the tool work out of the box for repos using 'master' or
other default branch names. Falls back to 'main' if detection fails.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: AlexFabre

commit_format/commit_format.py

@@ -392,6 +392,23 @@ def template_check(self, commit: str, commit_message: str) -> int:
         return errors
 
 
+def detect_base_branch() -> str:
+    """Auto-detect the default branch from the remote, fallback to 'main'."""
+    try:
+        result = subprocess.run(
+            ["git", "symbolic-ref", "refs/remotes/origin/HEAD"],
+            capture_output=True,
+            text=True,
+            check=False,
+        )
+        if result.returncode == 0:
+            # Output is like "refs/remotes/origin/main"
+            return result.stdout.strip().split("/")[-1]
+    except FileNotFoundError:
+        pass
+    return "main"
+
+
 def find_config_file() -> str | None:
     """Auto-discover config file in current directory or git root."""
     candidates = [".commit-format", ".commit-format.toml"]
@@ -449,12 +466,13 @@ def main():
         help="path to a commit-format template file to validate header/body/footer "
         "commit message structure",
     )
+    default_base = detect_base_branch()
     parser.add_argument(
         "-b",
         "--base",
         type=str,
-        default="main",
-        help="name of the base branch. Default 'main'",
+        default=default_base,
+        help=f"name of the base branch. Default '{default_base}'",
     )
     parser.add_argument(
         "-a",