add doc.go files; decompose agent package
doc.go:
- agent, backend, session, fs, bypass, toolclient (olliesrv)
- server, sandbox, registry (toolsrv)
- protocol, metadata (toolsrv shared)
- log, util, skills
agent package decomposition:
- chat.go: chat log, streaming, plan methods
- state.go: State, Reply, WaitChange, SignalCh, emit
- peer.go: AddPeer, RemovePeer, Peers
- subagent.go: Depth, IncChildren, DecChildren, ActiveChildren
- agent.go: 881 → 638 lines (core struct, identity, backend, runtime)
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