Add read-only execution eligibility surface

[m0r6aN]

Summary

• add a derived read-only execution eligibility substrate for prepared effects
• render eligibility on the prepared-effect inspection surface and as a compact chain step 8 summary
• add deterministic mock coverage plus tests for failure ordering and aggregate tone semantics

Verification

• pnpm exec eslint src/lib/collective/repositories.ts tests/unit/lib/collective/observability.test.ts
• pnpm test:unit tests/unit/lib/collective/observability.test.ts tests/unit/lib/collective/chain.test.ts