Adopt the standard repository release contract

[codeforester]

Goal

Make base-bash-libs an explicit consumer of the shared Base release standard so agents can follow the complete release sequence without relying on maintainer memory.

Scope

• Add the release: section to base_manifest.yaml.
• Add agent-facing docs/release-process.md for library releases.
• Document the version, changelog, tag, GitHub Release, Homebrew formula, Base dependency pin, validation, verification, and cleanup handoffs.
• Keep ordinary PRs from changing VERSION; release-prep PRs own version metadata updates.
• Validate the new contract with the existing library validation suite and Base release tooling.

Acceptance criteria

• basectl release check|plan|notes can operate against the library manifest.
• The documented process covers the library release and the downstream homebrew-base and Base CI updates.
• The release documentation is discoverable from repository agent guidance.
• No release is published as part of this issue.