Add generic GitHub CLI Bash helpers

[codeforester]

Summary

Add a small reusable GitHub CLI helper library for shell scripts that need gh readiness checks and bounded diagnostics without depending on Base.

Scope

• Add lib/bash/gh/lib_gh.sh with the standard lib_std.sh guard.
• Provide generic command availability, gh auth status -h github.com diagnostics, command failure reporting, and a thin gh runner.
• Add BATS coverage with fake gh commands for installed, missing, auth failure, command failure, and success cases.
• Document the helper in lib/bash/gh/README.md.

Non-goals

• No Base branch naming, issue categories, Project metadata, repo baseline policy, or generated PR bodies.
• Do not make base-bash-libs depend on Base.

Validation

• Focused BATS for the new gh library.
• ./tests/validate.sh.