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

[vivekchand]

Closes #601

What

Add support for detecting and displaying openclaw:prompt-error events throughout ClawMetry:

• New /api/prompt-errors endpoint for querying prompt errors
• /api/prompt-errors/alert endpoint for banner display on overview
• promptErrorsLast24h field in /api/overview response
• PROMPT_ERROR events now emitted in brain-stream SSE for real-time monitoring
• promptErrors field in brain-history response

How

• Scan session JSONL files for custom events with customType: openclaw:prompt-error
• Extract error details: provider, model, error message, timestamp, sessionId
• Sort by time descending, limit to recent 50-100 events
• Add red (#ef4444) color coding for PROMPT_ERROR events in stream

These are real provider failures: rate limits, auth issues, context overflow, model-not-found — now visible in the dashboard instead of ignored.

[vivekchand]

Closing as superseded by #797, which addresses the same issue (#601) with a cleaner implementation and fixes the CI failures present in this PR.