fix: parse LEAN_VERSION_MINOR correctly in release_checklist.py

The CMakeLists.txt format is `set(LEAN_VERSION_MINOR 30 CACHE STRING "")`,
so taking `split()[-1]` gives `""` not the version number. Use a regex to
extract the digit directly.

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

script/release_checklist.py

@@ -1004,7 +1004,8 @@ def main():
         # Find the actual minor version in CMakeLists.txt
         for line in cmake_lines:
             if line.strip().startswith("set(LEAN_VERSION_MINOR "):
-                actual_minor = int(line.split()[-1].rstrip(")"))
+                m = re.search(r'set\(LEAN_VERSION_MINOR\s+(\d+)', line)
+                actual_minor = int(m.group(1)) if m else 0
                 version_minor_correct = actual_minor >= next_minor
                 break
         else: