docs: close runtime-evidence product-readiness contract gap

[bateau84]

Product Readiness finding

PR #45 makes runtime_evidence mandatory and the host CLI rejects a result that omits or violates opencode-eval-runner/runtime-evidence/v1, but the main README's Result contract example still showed the older shape without that object. The compatibility consequence for legacy/custom images was also not stated.

That leaves the primary consumer-facing contract contradictory even though the runtime implementation is correct.

Changes

• make runtime_evidence explicit in the README Result contract example;
• state that every official transport result carries it, with Copilot reporting unsupported;
• document that host runner + overridden/custom image are a compatibility pair and must be upgraded together;
• add the assertion-level consumer rule: required boundaries must be complete and required exact fields available; diagnostic convenience fields cannot fill gaps;
• document the current aggregate and per-field capture bounds and their fail-closed meaning.

Scope

Documentation only. No runtime behavior, OpenCode patching, evidence channel redesign, signing, broker, PluginHost, or hostile-plugin isolation changes.

Targets refactor/trusted-checkout-evidence and is based directly on reviewed PR #45 head e3eb017ff070f3956119472a7d0fb74faa247102.