Clarify script freshness checks

[codeforester]

Summary

• Keep check_script_up_to_date local-ref-only by default and log that freshness contract.
• Add --fetch to refresh upstream state before comparing when callers want a live remote check.
• Document the freshness behavior and add stale-ref/fetch BATS coverage.

Validation

• RED: bats --filter "check_script_up_to_date (uses local|fetches)" lib/bash/git/tests/lib_git.bats failed before the fix because local-ref messaging and --fetch support were missing.
• GREEN: bats --filter "check_script_up_to_date (uses local|fetches)" lib/bash/git/tests/lib_git.bats
• bats lib/bash/git/tests/lib_git.bats
• shellcheck --severity=error lib/bash/git/lib_git.sh lib/bash/git/tests/lib_git.bats
• ./tests/validate.sh
• git diff --check

Fixes #13