Adds five telemetry events to track subagent lifecycle and user
interactions:
- **Subagent Started** — fires on creation or resumption, includes
parent session, subagent session, depth, and whether it was resumed
- **Subagent Completed** — fires when `SpawnAgentTool` finishes,
includes subagent session and status (completed/error)
- **Subagent Toggled** — fires when the user expands or collapses a
subagent card
- **Subagent Stopped** — fires when the user clicks Stop on a running
subagent
- **Subagent Maximized** — fires when the user clicks Maximize to
navigate into a subagent
Release Notes:
- N/A