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

[vivekchand]

Closes #601

What

Adds endpoint /api/prompt-errors to scan session JSONLs for openclaw:prompt-error custom events. Returns error details including model, provider, retries, and error message.

Includes a banner field in /api/overview response for the UI to display a warning when prompt errors are detected in recent sessions.

How

• New route /api/prompt-errors in routes/alerts.py scans the last 20 session JSONL files for openclaw:prompt-error events
• Returns structured error info: timestamp, session_id, error, model, provider, retries
• Builds a banner object for the most recent error to enable UI warnings
• Updated /api/overview in routes/overview.py to scan for prompt errors and include a banner field when errors are found

[vivekchand]

Closing this PR as it has been superseded by #788, which provides a more comprehensive implementation of the same feature (openclaw:prompt-error events integration). #788 includes brain.py integration with real-time SSE monitoring and more detailed error tracking.\n\nThank you for the contribution!