feat(mirrors): support URL-only MIRRORS entries and keep git config clean

- Allow MIRRORS to contain plain URLs (one per line) in addition to legacy "NAME URL"
- Treat strings as single URLs to avoid iterable pitfalls
- Write PyPI URLs as metadata-only entries (never added to git config)
- Keep MIRRORS as the single source of truth for mirror setup
- Update integration test to assert URL-only MIRRORS output

https://chatgpt.com/share/6941a9aa-b8b4-800f-963d-2486b34856b1

tests/integration/test_repos_create_pypi_not_in_git_config.py

@@ -75,12 +75,12 @@ class TestCreateRepoPypiNotInGitConfig(unittest.TestCase):
 
             mirrors_content = mirrors_file.read_text(encoding="utf-8")
             self.assertIn(
-                "pypi https://pypi.org/project/repo/",
+                "https://pypi.org/project/repo/",
                 mirrors_content,
                 "PyPI mirror entry must exist in MIRRORS",
             )
             self.assertIn(
-                "origin git@github.com:acme/repo.git",
+                "git@github.com:acme/repo.git",
                 mirrors_content,
                 "origin SSH URL must exist in MIRRORS",
             )