Enforce Fallow architecture boundaries

[itzcull]

Summary

• add custom Fallow zones and rules for the hexagonal architecture
• fail Fallow boundary violations in CI while keeping unused dependency warnings unchanged
• document boundary validation commands and Fallow CI coverage in README and AGENTS.md

Verification

• pnpm fallow list --boundaries
• pnpm fallow dead-code --boundary-violations
• pnpm run ci
• pnpm vitest run --project unit