Add --on flag to gt sling for formula scaffolding (gt-8tmz.9)

When --on is specified, the first argument is a formula name:
  gt sling shiny --on gt-abc123

This applies the formula to existing work, creating wisp scaffolding
that shapes execution of the target bead.

Changes:
- Add Formula field to SlungWork wisp type
- Add --on flag to gt sling command
- Verify both bead and formula exist before slinging
- Update dry-run output to show formula info

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/wisp/types.go

@@ -53,6 +53,11 @@ type SlungWork struct {
 	// BeadID is the issue/bead to work on (e.g., "gt-xxx").
 	BeadID string `json:"bead_id"`
 
+	// Formula is the optional formula/form to apply to the work.
+	// When set, this creates scaffolding around the target bead.
+	// Used by `gt sling <formula> --on <bead>`.
+	Formula string `json:"formula,omitempty"`
+
 	// Context is optional additional context from the slinger.
 	Context string `json:"context,omitempty"`