Add AgentPlane runtime evidence emission stub

[mdheller]

Context

AgentPlane remains the durable execution/evidence plane. Agent Machine must emit or stage runtime evidence rather than asking AgentPlane to infer missing deployment metadata after the fact.

Required outcome

Define an AgentPlane runtime evidence emission stub for Agent Machine placement and activation.

Acceptance criteria

• Add a documented runtime evidence payload shape.
• Evidence includes AgentMachine ID, AgentPod ID, provider ID, deployment receipt ID, Policy Fabric decision reference, Agent Registry grant reference, image digest, model digest, tokenizer digest, storage receipt references, cache reuse decision, and runtime status.
• Evidence payload excludes raw prompts, raw KV-cache contents, private memory contents, unredacted credentials, and provider secrets.
• Evidence semantics distinguish placement evidence, activation evidence, runtime status evidence, and teardown/wipe evidence.
• Failure semantics state that sensitive activation fails closed if required evidence emission is unavailable.

Related docs

• docs/architecture/world-class-release-gate.md
• docs/architecture/receipt-chain.md