Run a Mini App agent turn in the runtime #258

Parent

#251

What to build

The runtime can run a Mini App's agent turn: per-plugin admission and queueing, turn lineage, host
sessions the plugin creates and drives, and the facade that fronts them — admitted only for a plugin
whose manifest grants that access.

Acceptance criteria

• A plugin that declares no agent access is denied agent calls; one declaring session run is
  admitted.
• Concurrency and queue bounds come from the manifest and are enforced, with a busy reason the
  caller can read.
• A turn runs to completion through the host session ports and returns its result.
• Permission and questionnaire paths reach the caller with their request identifiers intact.
• The application exposes the agent capability to the client facade without exposing the ports
  themselves.