Draft: add agent-plane artifact scaffolds

[mdheller]

What this lands

This draft PR adds additive artifact scaffolds under schemas/:

• session-artifact.schema.v0.1.json
• promotion-artifact.schema.v0.1.json
• reversal-artifact.schema.v0.1.json
• bundle.schema.patch.json

Why additive first

The current bundle schema and the existing run/replay artifacts remain untouched in this draft. The goal is to land the next receipt family and the explicit patch fragment for new spec fields without risking in-place schema churn before the sourceos-spec agent-plane objects settle.

What this enables

• a typed session artifact that can hang off the existing run/replay evidence flow
• explicit promotion and reversal artifacts instead of overloading replay semantics
• a concrete patch fragment showing the next runtime fields to wire into bundle.schema.v0.1.json

Follow-up queue

1. Update bundle.schema.v0.1.json itself with the new runtime fields.
2. Add emitters for session / promotion / reversal artifacts.
3. Align artifact refs with the new SessionReceipt and PromotionReceipt work in SourceOS-Linux/sourceos-spec.
4. Feed these artifacts into SociOS-Linux/imagelab validation and SocioProphet promotion/reversal flows.

Clean-room posture

This work is derived from the public repo surfaces in our stack plus public reporting about Claude Code leak-era lessons. It does not copy proprietary Anthropic implementation details.