agent simulate: read simulator flow nodes from scenarios.yaml

[chenghao-mou]

lk agent simulate reads simulator flows from scenarios.yaml: a top-level nodes map, and start and max_duration on a scenario. They go to the server in the ScenarioGroup added by livekit/protocol#1856.

nodes:
  menu:
    say: "For appointments, press 1."
    speaker: MACHINE
    routes: [{ match: "1", to: sam }]
    timeout: { after: 8s, to: livekit/voicemail }
  sam:
    instructions: You are Sam Lee. Confirm tomorrow's 10am cleaning.
scenarios:
  - label: menu-to-sam
    start: menu
    agent_expectations: Presses 1, then confirms the appointment with Sam.

• Each node is decoded with protojson, so field names, enum names, durations, and unknown keys behave exactly as on the server. An error names the node, for example node "menu": ....
• The CLI checks two things locally: a scenario sets exactly one of start and instructions, and start names a node or a built-in (livekit/hangup, livekit/voicemail, livekit/unavailable). The server compiles and validates the rest at CreateSimulationRun.
• The YAML export writes nodes, start, and max_duration back, and adding scenario ids leaves nodes untouched. Files without flows are unchanged.

Warning

Depends on livekit/protocol#1856; go.mod points at its branch until it ships.

Not in this PR: the public OpenAPI client in pkg/public/oapi, which regenerates after the protocol change ships. Copying a flow scenario from a run in the TUI still yields a stub without its nodes.

Addresses AGT-3725