[Chrisbryan17]

Closes #749.

Problem

The check-shebang-scripts-are-executable hook unconditionally uses Git's index, so it fails when invoked on files outside a Git worktree.

Change

• Preserve Git-index executable bits inside working trees.
• Fall back to filesystem executable permissions on POSIX when outside a Git worktree.
• Preserve unrelated Git errors and avoid assuming Unix executable semantics on Windows.
• Add regression coverage for missing Git metadata, permission combinations, and error paths.

Testing

• Affected tests: 12 passed.
• Complete tests directory: 407 passed, 13 skipped, 2 xfailed (Windows, Python 3.13, third-party pytest plugin autoload disabled).
• git diff --check: passed.
• POSIX filesystem behavior was simulated with monkeypatches; it was not tested on a native POSIX host.