code-mode: move cell state into library actor

[cconger]

A code-mode cell is a single JavaScript execution that can produce output, call tools, wait for asynchronous work, resume, or be terminated. This PR extracts the existing per-cell run loop into a dedicated actor that owns the cell’s lifecycle state. It is primarily an ownership change rather than a new lifecycle contract: existing behavior now has one clear implementation boundary.

Architecture

The session service remains responsible for session-wide concerns: allocating cell IDs, storing shared values, creating cells, and routing requests to them.

Once a cell is created, its execution state belongs to its actor. Callers interact with the actor through a handle. The actor receives two kinds of input: runtime events and control requests.

A single event loop serializes these inputs and applies the lifecycle rules. It tracks the current observer—the caller waiting for an update—along with accumulated output, outstanding callbacks, runtime state, yield deadlines, and termination progress. Observation, termination, completion, and cleanup therefore have one consistent owner.

When the runtime has no immediately runnable work and is waiting only on timers or tool results, the actor can return accumulated output and information about outstanding tool calls while keeping the cell available to resume. On completion or termination, it performs the appropriate callback cleanup before publishing the final result and removing the cell from the session.

A small host interface connects the actor to session-owned facilities such as tool dispatch, notifications, stored values, and final cell removal, keeping those responsibilities outside the actor itself.

Why

Previously, cell lifecycle state and coordination lived alongside session management. The actor boundary makes each cell a self-contained state machine with a single writer, while the service becomes a registry and adapter around it.

This makes lifecycle behavior easier to reason about and test in isolation. It also establishes a clean boundary for later changing where cells run or how they communicate without recreating their lifecycle rules.