Expose generic eval runtime

[olivermt]

Summary

• add AshLua.Eval as a generic runtime API for manifest resolution, docs, and script execution
• let eval surfaces resolve from namespace action labels and cache eval manifests with preload_eval_manifests!/1
• document using AshLua as a transport-agnostic runtime for MCP/custom LLM loops

Checks

• mix deps.get
• mix compile
• MIX_ENV=test mix test
• mix check --no-retry

Stacked on ash-project#5.

[olivermt]

Closing in favor of an upstream PR to ash-project/main.