Fix gh helper output-variable shadowing

[codeforester]

Problem

gh_detect_default_branch and gh_repo_default_branch use local variable names that can shadow caller-provided output variable names. A caller passing default_branch as the result variable receives an empty value even when detection succeeds.

This was found while consuming the new helpers from Base issue #1488.

Scope

• Rename internal locals in the affected helpers so pass-by-name output works with common names such as default_branch.
• Add BATS coverage for output variables that match the former internal names.
• Keep the public helper contracts unchanged.

Acceptance Criteria

• gh_detect_default_branch <repo_dir> default_branch sets the caller variable.
• gh_repo_default_branch <repo> default_branch sets the caller variable.
• bats lib/bash/gh/tests/lib_gh.bats passes.
• tests/validate.sh passes.