feat(simulate): generate a scenario from a recorded session

[u9g]

Adds lk agent simulate scenario generate --from-session SESSION_ID, which turns a recorded session into a scenarios.yaml document that lk agent simulate --scenarios can run.

lk agent simulate scenario generate --from-session RM_xxx > scenarios.yaml
lk agent simulate text --scenarios scenarios.yaml

• The backend reads the session's chat_history recording, so the session must have been recorded (project recording setting on, or session.start(record=True)).
• Works with API keys (AgentSimulation.CreateScenarioFromSession via server-sdk-go) and under --experimental-auth (Public API POST /v0/projects/{projectId}/scenarios:generateFromSession).
• --json prints the scenario as the API returns it.
• Separate commit: generated scenarios YAML no longer emits name: "", tags: {} and userdata: {} when they are empty. This also applies to the scenarios file saved after a generated-scenarios run.

Tested end to end against a production project: generated a scenario from a recorded session, then ran it with lk agent simulate text --scenarios (1 passed).

Known issue outside this PR: production public-api-server is still on v2026-10-13-1825 (595078c), where some pods serve simulations in the stubbed grpc mode, so every Public API simulation endpoint (including simulate list/get) currently returns 501 service operation not implemented yet under --experimental-auth. The API-key path is unaffected. Deploying public-api-server main (≥ e43438e) fixes it.

[u9g]

Demo: lk agent simulate scenario generate --from-session turns a recorded production session into a scenarios.yaml, which then runs as-is with lk agent simulate text --scenarios (1 passed).

scenario-generate.mp4