Turn tracking for plugin prompt submissions now uses the real turn id

What

• New turn-lifecycle bookkeeping marks prompt UUIDs against turn.start/turn.complete events and prunes stale entries. The plugin API for submitting a prompt now uses this bookkeeping when submitting, tagging the submission to the current turn only if its origin is undefined or session.start.
• The handler that reports a completed agent turn to telemetry/session state now forwards the actual turnId from the event, rather than generating a new id at report time.

Why

This makes turn tracking for plugin-submitted prompts, and the turn id used in turn-complete reporting, more accurate and consistent instead of relying on freshly generated ids.