trigger: detect default branch from repo instead of hardcoding master

src/gitea_ci/commands/workflows.py

@@ -27,7 +27,10 @@ def cmd_trigger(api: GiteaAPI, args: argparse.Namespace) -> int:
         return 1
 
     workflow = args.workflow
-    ref = args.ref or "master"
+    ref = args.ref
+    if not ref:
+        repo_info = api.get_repo(owner, repo)
+        ref = (repo_info or {}).get("default_branch", "main")
 
     # Parse inputs if provided
     inputs = {}