Adopt Apache-2.0 license and refresh README entry point

[codeforester]

Context

base-bash-libs is now a generic reusable Bash library, not only an internal Base extraction. The repository still uses AGPL-3.0-or-later and the top-level README does not surface release metadata or link readers directly to the individual library READMEs.

Scope

• Replace the repository license with Apache-2.0.
• Update top-level README metadata so the current version, license, release notes, and installation path are visible near the top.
• Link the top-level README to the stdlib, file, and git library READMEs.
• Improve the README structure where it helps generic library consumers understand adoption paths.
• Add validation that keeps README release metadata aligned with the root VERSION file.
• Update CHANGELOG.md for the notable license/documentation change.
• Update Homebrew tap metadata after the source repo change lands.

Acceptance Criteria

• Repository LICENSE is Apache-2.0.
• README top strip shows the same version as VERSION.
• README library list links to the individual library README files.
• README remains clear for Homebrew, source checkout, vendored, and Base-managed workflows.
• Validation fails if README release metadata drifts from VERSION.
• Homebrew formula license metadata matches the repository license.

Validation

• ./tests/validate.sh
• git diff --check
• Homebrew formula validation after the source PR merges

Suggested Branch

enhancement/<issue>-20260618-apache-readme-refresh