Adds a new "Agent Panel Error Shown" telemetry event that fires when
users see errors in the agent panel. For errors from external ACP agents
(like Claude Code), we capture additional details.

Previously, we had no visibility into what errors users were
encountering in the agent panel. This made it difficult to diagnose
issues, especially with external agents.

The new telemetry event includes:
- `agent` — The agent telemetry ID
- `session_id` — The session ID
- `kind` — Error category (payment_required, authentication_required,
refusal, other, etc.)
- `acp_error_code` — The ACP error code when available (e.g.,
"InternalError")
- `acp_error_message` — The ACP error message when available

Release Notes:

- N/A

---------

Co-authored-by: Michael Benfield <mbenfield@zed.dev>