Add on_child_initialized callback to PreparedBundle.spawn()

[michaeljabbour]

Summary

Adds optional on_child_initialized callback parameter to PreparedBundle.spawn() that is invoked after child session initialization but before execution begins.

This enables apps (like Kepler) to inject hooks (streaming, observability, approval) on child sessions without inlining spawn internals or creating maintenance burden as foundation evolves.

Changes

• Add on_child_initialized: Any = None parameter to spawn() signature
• Invoke callback after child_session.initialize() and before instruction injection
• Callback receives initialized AmplifierSession for clean hook injection

Impact

Enables clean elimination of ~60 lines of inlined spawn logic in desktop apps that need child-session hooks. Any app requiring post-initialization, pre-execution injection gets the same benefit.

Example

prepared.spawn(
    instruction=task,
    on_child_initialized=lambda session: bridge.register_on_coordinator(
        session.coordinator, is_child=True, agent_name=agent
    )
)