governance: codify delivery cycle and repo fixture testing strategy

[flyingrobots]

Summary

Codify Git Mind's new delivery cycle and tests-as-spec workflow so contributors and agents follow a consistent IBM Design Thinking-driven loop.

Scope

• establish the delivery cycle as canonical repo governance
• define tests-as-spec expectations
• define repo fixture strategy (base repos + scenario overlays, tarballs only when needed)
• update visible contributor and canonical docs

Why

We want the first implementation cycles for Hill 1 to start from a shared execution model instead of improvising process and test substrate while building product code.