Define Portable AI runtime receipt integration

[mdheller]

Summary

Adds the Agent Machine runtime integration spec for SourceOS Portable AI Kit.

This defines how a USB/SSD-local portable AI root becomes a governed runtime substrate with:

• preflight receipts;
• activation receipts;
• teardown receipts;
• wipe receipts;
• provider order;
• surface handoff;
• zero-trace audit posture;
• activation failure gates.

Product target

Match the usability of plug-in local AI kits while making runtime activation, storage locality, cache behavior, host writes, and shutdown auditable by Agent Machine, Policy Fabric, Agent Registry, and AgentPlane.

Follow-up acceptance criteria

• Add receipt schemas.
• Add examples for portable runtime preflight/activation/teardown/wipe.
• Add agent-machine render portable-runtime-plan or equivalent.
• Wire dry-run activation readiness evaluation.