Add spawned_id to spawn_agent and allow main agent to resume it

[zikajk]

Add smething like spawned_id to subagent which will effectively resume the chat that has been created with another subagent in the session.
And add something like this into description of the spawn_agent tool:
Each agent invocation starts with a fresh context unless you provide spawned_id to resume the same subagent session (which continues with its previous messages and tool outputs). When starting fresh, your prompt should contain a highly detailed task.

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