docs: define deterministic fixture contract (DEN-613)

[ORESoftware]

Implements the documentation/policy portion of DEN-613 for node-app.

• classifies the repo explicitly as a deterministic interoperability fixture, not production
• documents its Zed/npm package graph and deterministic inputs/outputs
• records local symlink vs Docker/OCI copy-mode expectations without duplicating DEN-588/DEN-591 canary work
• documents integrity/filesystem-boundary failure expectations
• adds lowercase agents.md with automation and inherited security/contact rules

Follow-up under DEN-613 should add/verify the consuming CI assertions and apply the same policy rollout across the other canonical fixtures.

[linear-code]

DEN-613