docs(resources): define owner-scoped recoverable references

[627150795]

Problem

OpenPI produces Subagent, Workflow, Background Terminal, Session, and external resources, but their references have different identities, owners, retention, and recovery guarantees. A process-global router would risk crossing those boundaries.

Value

This defines a thin, owner-scoped reference contract for exact artifacts, bounded projections, persistence, replay, branches, permissions, invalidation, and typed failures. It makes recovery paths explicit without treating paths or IDs as capabilities.

Approach

Added a focused design document and linked it from the design index. The contract keeps each existing owner responsible for storage and resolution, treats resolved content as untrusted data, and recommends shared validation only when multiple owners need the same serialization.

Validation

• git diff --cached --check
• bun run lint
• bun run typecheck

Impact

Documentation-only; no resource router, API, UI, artifact store, replay, or runtime behavior changes.

Fixes #157

[tt-a1i]

Thank you for preserving this research. We are closing this PR as a source record for the Capability, authority, and resources consolidation tracked in #277. This is not a finding that the content is wrong. The current queue contains overlapping contracts from one stale base, so we will extract only settled boundaries into at most one successor record, citing this PR explicitly. Please do not spend time rebasing this branch.