chore: Drop JSON formula support, TOML only (gt-spt8v)

- Delete all .formula.json files (TOML versions exist)
- Update sling.go comment to say TOML only

Note: Doc updates were prepared but those docs were deleted
upstream in the recent refactoring.

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

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

internal/cmd/sling.go

@@ -469,7 +469,7 @@ func resolveSelfTarget() (agentID string, pane string, hookRoot string, err erro
 }
 
 // verifyFormulaExists checks that the formula exists using bd formula show.
-// Formulas can be formula files (.formula.json/.formula.toml).
+// Formulas are TOML files (.formula.toml).
 func verifyFormulaExists(formulaName string) error {
 	// Try bd formula show (handles all formula file formats)
 	cmd := exec.Command("bd", "formula", "show", formulaName)