Define check_script_up_to_date return precedence for behind and dirty scripts

[codeforester]

Context

check_script_up_to_date can detect both of these states:

• repository is behind upstream, returned as status 2
• tracked script has local modifications, returned as status 3

The current implementation returns 2 as soon as it detects the repo is behind, so status 3 is unreachable when both conditions are true.

Failure Scenario

A repo is behind origin/main and the checked script has local modifications. The helper logs the dirty warning earlier, then returns 2 for behind. Automation that branches on status 3 cannot detect the local modification state.

Scope

• Define the public return-code precedence for combined behind and dirty states.
• Implement the chosen behavior without losing useful log output for either condition.
• Document return statuses in lib/bash/git/README.md.
• Add BATS coverage for the combined behind plus dirty case.

Acceptance Criteria

• The combined state has a documented and tested return status.
• Existing up-to-date, behind-only, and dirty-only tests continue to pass or are updated to match the documented contract.
• ./tests/validate.sh passes.

Validation

• ./tests/validate.sh