Refocus the Astro build guide

[jmchilton]

Summary

• refocus Build with the Astro Stack on repository-level composition instead of defending the stack choice
• define the ownership seam among domain knowledge, instance policy, shared foundry-lib mechanics, and generated outputs
• replace implementation-history prose with a compact responsibility matrix and explicit documentation handoffs
• add a responsive architecture diagram and mobile-friendly matrix

Validation

• npm test
• npm run build
• visually checked the page at desktop and narrow layouts

Paired documentation

• Link package docs to the Pattern build guide jmchilton/foundry-lib#57 adds the reciprocal package-level links and establishes the canonical home for package behavior and APIs.