Agent-template encoders mirror the on-chain Sigil decoder byte layout

Decision

The agent template’s encode_intent_envelope and encode_action_envelope produce bytes that match the on-chain Sigil decode_intent and decode_action exactly. Three unit tests confirm minimum length, fixed action size, and oversize-venue rejection.

Consequences

Future drift on either side fails the agent-template unit tests rather than producing an envelope the decoder silently rejects.