Document delegation-first manifest design

[codeforester]

Summary

• Reframe base_manifest.yaml docs around delegation-first orchestration.
• Document Brewfile as the preferred path for ordinary Homebrew formulae and casks.
• Describe planned mise and test manifest contracts without implementing parser behavior yet.
• Keep the artifact registry positioned as small and Base-aware, with Python packages still owned by Base.
• Split the broad TODO into focused follow-ups for mise, basectl test, structured python:, and setup hook design.

Validation

• git diff --check