Adopt main as base-bash-libs default branch

[codeforester]

Context

Use main consistently across Base-family repositories. The local base-bash-libs checkout currently tracks origin/master, and this repo still contains master references in docs and git-helper tests.

Examples found locally include STANDARDS.md links to base/blob/master, lib/bash/git/lib_git.sh fallback behavior, lib/bash/git/tests/lib_git.bats, and shared test helpers that initialize fixture repositories on master.

This should coordinate with basefoundry/base#904.

Scope

• Rename the GitHub default branch from master to main once the compatibility plan is ready.
• Update documentation links that point at base/blob/master to use main or a more durable reference.
• Make git helper fixtures and messages main-first where they represent the normal default branch.
• Preserve explicit coverage for repositories that still use master, since the library should remain compatible with legacy repos.
• Verify release and Homebrew consumers after the rename.

Acceptance criteria

• GitHub reports main as the default branch for base-bash-libs.
• User-facing docs no longer point readers at master as the normal branch.
• Tests distinguish the normal main path from the legacy master fallback path.
• ./tests/validate.sh passes after the rename and reference updates.