Draft: add agent-plane capability patch fragment

[mdheller]

What this lands

This draft PR adds an additive capability-descriptor patch fragment:

• capd/imagelab.capd.patch.yaml

Why additive first

The current descriptor stays intact. This file captures the next contract directories and validator entrypoints needed once the new agent-plane schema family and receipts are accepted upstream.

What it describes

• session / skill / memory contract directories
• validator entrypoints for ExecutionDecision, SkillManifest, and SessionReceipt

Follow-up queue

1. Fold these fields into the canonical imagelab.capd.v0.yaml once the upstream spec family settles.
2. Add the actual validator modules under validators/.
3. Wire schema indexing and admission checks to the new agent-plane objects.

Clean-room posture

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