Reindex completed with errors

### Describe the bug

Runing `/chronicle reindex` shows:

> ● ⚠ Reindex completed with errors: 5 sessions indexed locally, 41 queued for backfill. Errors: 2 session(s) failed

with no explanation of what the errors are, where I can find more information or what I can do to remedy the situation.

### Affected version

GitHub Copilot CLI 1.0.62.

### Steps to reproduce the behavior

1. Type `/chronicle reindex`
2. Observe errors

### Expected behavior

An explanation of what the errors are, where I can find more information or what I can do to remedy the situation.

### Additional context

_No response_

Start by tracing the `/chronicle reindex` command entry point and its error-reporting path. Reproduce the command and inspect how failed sessions are handled. Done means the output explains the errors, points to more information, and indicates possible remedies.

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