Harden named-output helper locals

[codeforester]

Summary

• Harden named-output helpers so caller variables such as result_name, temp_path, options_name, and result_var are not shadowed by helper-local variables.
• Add regression coverage across std, string, arg, git, and GitHub helper APIs.
• Document the release-worthy fix in CHANGELOG.md.

Validation

• bats lib/bash/str/tests/lib_str.bats lib/bash/std/tests/lib_std.bats lib/bash/git/tests/lib_git.bats lib/bash/gh/tests/lib_gh.bats lib/bash/arg/tests/lib_arg.bats
• git diff --check
• ./tests/validate.sh

Fixes #137