When using `gt sling <formula> --on <bead>`, the code was only passing
the `feature` variable (set to bead title). This broke formulas that
expect `issue` (set to bead ID), like mol-polecat-work.
Now passes both common variables:
- feature: bead title (for shiny-style formulas)
- issue: bead ID (for mol-polecat-work-style formulas)
This allows either formula type to work with --on without requiring
the user to manually specify variables.
Fixes#355
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>
a1008f6f58