fix(sling): pass issue variable with bead ID instead of feature with title

When using `gt sling <formula> --on <bead>`, the wisp creation was
incorrectly passing `feature=<title>` instead of `issue=<beadID>`.
This caused formulas like mol-polecat-work to fail with "missing
required variables: issue".

Fixes: gt-ojnm2

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

Author: furiosa

internal/cmd/sling.go

@@ -400,9 +400,9 @@ func runSling(cmd *cobra.Command, args []string) error {
 			return fmt.Errorf("cooking formula %s: %w", formulaName, err)
 		}
 
-		// Step 2: Create wisp with feature variable from bead title
-		featureVar := fmt.Sprintf("feature=%s", info.Title)
-		wispArgs := []string{"--no-daemon", "mol", "wisp", formulaName, "--var", featureVar, "--json"}
+		// Step 2: Create wisp with issue variable from bead ID
+		issueVar := fmt.Sprintf("issue=%s", beadID)
+		wispArgs := []string{"--no-daemon", "mol", "wisp", formulaName, "--var", issueVar, "--json"}
 		wispCmd := exec.Command("bd", wispArgs...)
 		wispCmd.Dir = formulaWorkDir
 		wispCmd.Stderr = os.Stderr