Practical guidance - Keep prompts off-chain and $0 by defau…

Practical guidance
- Keep prompts off-chain and $0 by default (signed envelopes).
- Use on-chain only for: key/session management, rare strict anti‑replay (on‑chain nonce), and optional audit events. Do these on an L2 to make them pennies.
- Tools the agent calls should verify scopes against the same registry to neutralize prompt‑injection attempts.