Clarify freshness semantics for check_script_up_to_date

[codeforester]

Context

check_script_up_to_date compares local HEAD with the configured upstream ref, but it does not fetch first. If remote-tracking refs are stale, the function can report up to date when the remote has moved.

Fetching unconditionally may be undesirable in offline or latency-sensitive scripts, so this needs an explicit API/product decision.

Scope

• Decide whether check_script_up_to_date should fetch by default, fetch behind an option/env var, or explicitly remain a local-ref-only check.
• Implement the chosen behavior in lib/bash/git/lib_git.sh.
• Update lib/bash/git/README.md to describe the freshness contract.
• Add BATS coverage for the selected behavior.

Acceptance Criteria

• Callers can tell whether the check reflects live remote state or only local remote-tracking refs.
• Stale-ref behavior is tested or documented as intentional.
• Existing skip conditions for no git, no upstream, untracked scripts, and non-repo paths remain predictable.

Validation

• ./tests/validate.sh

Suggested Branch

enhancement/<issue>-20260618-script-freshness-contract