file-issues: auto-create missing labels instead of failing

[benthomasson]

Summary

code-expert file-issues fails for every issue when the target repo doesn't have the required labels (reasons-gate, reasons-negative). The gh issue create call errors with could not add label: 'reasons-gate' not found and the entire batch produces 0 filed issues.

Expected behavior

Before filing issues, file-issues should check whether the required labels exist on the target repo and create any that are missing (via gh label create). This way the command works out of the box against any repo without manual label setup.

Current behavior

All issues fail with label-not-found errors. The user must manually run gh label create for each label before retrying.

Suggested fix

Before the filing loop, add a label-ensure step:

1. List existing labels on the target repo (gh label list)
2. For any required label not present, create it (gh label create)
3. Proceed with filing

This is a one-time check per invocation, not per-issue.