fix(acp): surface session-busy as a typed JSON-RPC error instead of -32603

Autonomous turns stream without an owning promptTurn, so the implicit-cancel
guard in prompt() cannot fire; a client session/prompt landing mid-turn fails
inside AgentSession.prompt with a raw AgentBusyError that transport.ts wraps
into a generic -32603 "Internal error". Clients cannot tell busy from broken,
so user prompts appear to vanish (can1357/oh-my-pi#9157).

Map AgentBusyError onto RequestError(-32003) with data
{ reason: "session_busy", hint: "steer|followUp|wait" } at the single
prompt-path catch; the message keeps AgentBusyError's own wording so TUI and
ACP stay consistent.