session: remove Submit and agent pass-through methods
Session no longer has Submit(), IsRunning(), State(), Reply(), BackendName(),
ModelName(), AgentName(), CtxSz(), Cost(), Usage(), Context(), SystemPrompt(),
GenerationParams(), SetGenerationParams(), CompactionModel(), SetCompactionModel(),
ListModels(), Reactions(), React(), ReactTo(), PopQueue(), ToolCallCount().
Callers access the Agent directly via Session.Agent(). Session retains only
genuinely session-level operations: Agent(), Bus(), Close(), SetEnv(),
Interrupt(), Queue(), InjectSystemEvent(), CWD(), SetCWD(), SetSessionID(),
WaitChange(), persistence, and detach.
Agent.Submit now dispatches slash commands internally (HandleCommand) before
falling through to executeTurn.
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