[codex] Add surface contract view

[nahiyankhan]

Summary

Adds a read-only Surface Contract View layer that derives from the host-selected compiled SurfacePolicy and makes Summon's typed boundaries explicit without changing enforcement.

What changed

• Added public SurfaceContractView types and helpers in engine/public facades.
• Added a concise surface-contract system prompt block while preserving detailed capability and component prompt blocks.
• Emits host-owned /surface-contract meta after /surface-policy and /surface-plan for policy-backed generation sessions.
• Blocks model-authored /surface-contract like the other host-owned surface meta paths.
• Wires Devtools, Generate workbench, and Surface Gallery summaries to prefer the emitted view.
• Completes empty-state key follow-through in validator/host resource maps so the current typed contract remains coherent.

Validation

• pnpm test
• pnpm test:gallery
• pnpm test:safety
• pnpm build:impl
• pnpm build:public-core
• pnpm check:public-api
• git diff --check