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

Builds on #5; once that merges, this PR should reduce to the follow-up eval/runtime commit.

[zachdaniel]

That PR is merged, not sure what this changes (or really why, as the description is light on that context), can you pull into this branch?

[olivermt]

I thought there was a dependency but I can't find it now, so to be honest I am not quite sure what I meant :S
PR is good to go from my side as-is!
I also rebased in the conflicts.