Document the base-bash-libs release process

[codeforester]

Summary

• Declare the machine-readable release contract for base-bash-libs in base_manifest.yaml.
• Document the standard release ceremony, Homebrew handoff, and Base full-SHA dependency handoff.
• Add agent guidance and contributor/README links so release work follows the repository contract.

Validation

• ./tests/validate.sh
  • 301 BATS tests passed
  • Bash library validation passed.
• BASE_BASH_LIBS_DIR=/Users/rameshhp/work/base-bash-libs/lib/bash basectl release plan --version 1.3.0 --manifest base_manifest.yaml
• git diff --check

Notes

• Ordinary pull requests leave VERSION unchanged.
• The Homebrew formula and Base dependency pin remain downstream release handoffs; this PR does not publish a release.

Closes #181