feat: Wire openclaw:prompt-error events into alerts stream

[vivekchand]

Closes #601

What

This PR wires openclaw:prompt-error custom events into the ClawMetry alerts stream. These events indicate real provider failures — rate limits, auth issues, context overflow, model-not-found — that are currently being ignored.

How

1. Modified _parse_jsonl_event() in the brain stream to detect custom type events with customType: "openclaw:prompt-error"
2. Convert these events to ERROR-type brain events with red color (#ef4444)
3. When emitting events, fire a ClawMetry alert for any ERROR event containing "prompt-error"
4. Alerts use rule_id="prompt-error-detector", severity="error", and display via the banner channel

Testing

• Syntax check passed: python3 -m py_compile dashboard.py
• The implementation follows existing patterns in the codebase for custom event handling

Scope

• Self-hosted mode: reads errors directly from JSONL files
• Cloud mode: will pick up the UI changes (already streams via encrypted events)

[vivekchand]

✅ This feature has been superseded by PR #788 which implements the same functionality. The openclaw:prompt-error events are now properly wired into the alerts stream. Closing as superseded.

[vivekchand]

Closing as the changes are already implemented in a newer PR #788