docs: add IFC and negotiation architecture notes

[tobytkershaw]

Summary

• add an IFC reintegration proposal that positions IFC as the default out-of-vault communication membrane
• add a negotiation protocol sketch centered on bounded-computation agreements and resolved execution contracts
• add lightweight notes for negotiation registry artefacts, wire format, and example fixtures

Notes

• docs-only change
• architecture sketches, not yet normative specs