docs: define Pi-first agent contract

[tt-a1i]

Summary

• make AGENTS.md the canonical Pi-first, model-amplifying design contract
• separate model judgment from enforceable runtime invariants
• document progressive disclosure, Pi-native composition, and non-blocking subagent guidance
• add a minimal CLAUDE.md adapter that points to the shared contract
• preserve the existing repository, type-safety, child-tool, and /openpi-setup configuration rules

Closes #34

Validation

• bun run check
• bun run test (Node 719/719, Vitest 29/29)