## Summary
- introduce a private `session_runtime` boundary for cell creation
requests, observation modes, lifecycle events, output items, and tool
metadata
- update the cell actor and in-process service to use those
transport-neutral types
- keep cell ID allocation on the owning session side
## Motivation
Cell lifecycle vocabulary currently lives inside the cell actor
implementation. That makes the service adapter and future session
runtime depend on actor-specific types, increasing the size and
complexity of the runtime ownership change.
This is the first reviewable slice of the session-runtime stack. It
separates the transport-neutral data model without moving lifecycle
ownership or changing behavior.
Later slices will move session state behind this boundary, harden
terminal and shutdown behavior, and split cell creation from
observation.
## Behavior
There are no public API or user-visible behavior changes in this PR.
In particular:
- `CodeModeSession::execute` and `wait` are unchanged
- cell IDs remain allocated by the owning session
- cell admission, observation, termination, and shutdown behavior are
unchanged
Channing Conger
6d993ca646