Tracks Legion agent invocations and their telemetry events.

Maintains three ETS tables:

• :legion_web_agents — one record per tracked agent, keyed by agent_id
• :legion_web_events — ordered event log per agent, keyed by {agent_id, seq}
• :legion_web_usage — ordered LLM usage entries per agent, keyed by {agent_id, seq}

Attaches to Legion and dashboard telemetry events on startup. Broadcasts changes via LegionWeb.PubSub so LiveView subscribers receive real-time updates.

Usage is recorded only while config :legion, :track_usage is enabled (the default), matching what Legion itself persists.