Fix PR policy target ref resolution

ci/check_pr_policies.py

@@ -22,8 +22,45 @@ TEST_PATTERNS = [
 def run(cmd: str) -> str:
     return subprocess.check_output(cmd, shell=True, text=True).strip()
 
+def git_ref_exists(ref: str) -> bool:
+    try:
+        subprocess.check_call(
+            f"git rev-parse --verify --quiet '{ref}'",
+            shell=True,
+            stdout=subprocess.DEVNULL,
+            stderr=subprocess.DEVNULL,
+        )
+        return True
+    except subprocess.CalledProcessError:
+        return False
+
+def resolve_target_ref() -> str:
+    raw = os.environ.get("CHANGE_TARGET", "").strip()
+
+    candidates = []
+    if raw:
+        candidates.append(raw)
+        if not raw.startswith("origin/"):
+            candidates.append(f"origin/{raw}")
+
+    candidates.extend([
+        "origin/main",
+        "main",
+    ])
+
+    seen = set()
+    for ref in candidates:
+        if ref and ref not in seen:
+            seen.add(ref)
+            if git_ref_exists(ref):
+                return ref
+
+    print("ERROR: Kein gültiger Ziel-Branch-Ref gefunden. Geprüft:", ", ".join(candidates))
+    sys.exit(1)
+
 def changed_files():
-    target = os.environ.get("CHANGE_TARGET", "origin/main")
+    target = resolve_target_ref()
+    print(f"Vergleiche gegen Ziel-Ref: {target}")
     out = run(f"git diff --name-only {target}...HEAD")
     return [line.strip() for line in out.splitlines() if line.strip()]