Session.Submit now forwards directly to Agent.Submit. Slash commands
are agent-level concerns handled by Agent.HandleCommand.
Session-level operations (save, resume, kill, rename, cwd) are ctl
verbs issued via the ctl file — they are NOT slash commands and do
not flow through the prompt path.
This eliminates the artificial session/agent command routing and
makes the prompt path clean: everything in the prompt goes to the agent.
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