code-mode: define transport-neutral runtime types

[cconger]

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