feat: Implement formula-on-bead mode in gt sling

When using `gt sling <formula> --on <bead>`, the command now properly
instantiates the formula as a molecule and bonds it to the target bead:

1. Cook the formula (ensures proto exists)
2. Create wisp with feature variable from bead title
3. Bond wisp to original bead (creates compound)
4. Hook the compound root (not bare bead) to target agent

This enables the "shiny" workflow (design→implement→review→test→submit)
to be applied to existing bugs/features, ensuring polecats complete all
phases including the code review step.

Example:
  gt sling shiny --on gt-abc gastown
  # Creates shiny molecule, bonds to gt-abc, slings compound to polecat

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

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

internal/cmd/sling.go

@@ -218,9 +218,14 @@ func runSling(cmd *cobra.Command, args []string) error {
 	}
 
 	if slingDryRun {
-		fmt.Printf("Would run: bd update %s --status=hooked --assignee=%s\n", beadID, targetAgent)
 		if formulaName != "" {
-			fmt.Printf("  formula: %s\n", formulaName)
+			fmt.Printf("Would instantiate formula %s:\n", formulaName)
+			fmt.Printf("  1. bd cook %s\n", formulaName)
+			fmt.Printf("  2. bd mol wisp %s --var feature=\"%s\"\n", formulaName, info.Title)
+			fmt.Printf("  3. bd mol bond <wisp-root> %s\n", beadID)
+			fmt.Printf("  4. bd update <compound-root> --status=hooked --assignee=%s\n", targetAgent)
+		} else {
+			fmt.Printf("Would run: bd update %s --status=hooked --assignee=%s\n", beadID, targetAgent)
 		}
 		if slingSubject != "" {
 			fmt.Printf("  subject (in nudge): %s\n", slingSubject)
@@ -235,6 +240,64 @@ func runSling(cmd *cobra.Command, args []string) error {
 		return nil
 	}
 
+	// Formula-on-bead mode: instantiate formula and bond to original bead
+	if formulaName != "" {
+		fmt.Printf("  Instantiating formula %s...\n", formulaName)
+
+		// Step 1: Cook the formula (ensures proto exists)
+		cookCmd := exec.Command("bd", "cook", formulaName)
+		cookCmd.Stderr = os.Stderr
+		if err := cookCmd.Run(); err != nil {
+			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{"mol", "wisp", formulaName, "--var", featureVar, "--json"}
+		wispCmd := exec.Command("bd", wispArgs...)
+		wispCmd.Stderr = os.Stderr
+		wispOut, err := wispCmd.Output()
+		if err != nil {
+			return fmt.Errorf("creating wisp for formula %s: %w", formulaName, err)
+		}
+
+		// Parse wisp output to get the root ID
+		var wispResult struct {
+			RootID string `json:"root_id"`
+		}
+		if err := json.Unmarshal(wispOut, &wispResult); err != nil {
+			return fmt.Errorf("parsing wisp output: %w", err)
+		}
+		wispRootID := wispResult.RootID
+		fmt.Printf("%s Formula wisp created: %s\n", style.Bold.Render("✓"), wispRootID)
+
+		// Step 3: Bond wisp to original bead (creates compound)
+		bondArgs := []string{"mol", "bond", wispRootID, beadID, "--json"}
+		bondCmd := exec.Command("bd", bondArgs...)
+		bondCmd.Stderr = os.Stderr
+		bondOut, err := bondCmd.Output()
+		if err != nil {
+			return fmt.Errorf("bonding formula to bead: %w", err)
+		}
+
+		// Parse bond output - the wisp root becomes the compound root
+		// After bonding, we hook the wisp root (which now contains the original bead)
+		var bondResult struct {
+			RootID string `json:"root_id"`
+		}
+		if err := json.Unmarshal(bondOut, &bondResult); err != nil {
+			// Fallback: use wisp root as the compound root
+			fmt.Printf("%s Could not parse bond output, using wisp root\n", style.Dim.Render("Warning:"))
+		} else if bondResult.RootID != "" {
+			wispRootID = bondResult.RootID
+		}
+
+		fmt.Printf("%s Formula bonded to %s\n", style.Bold.Render("✓"), beadID)
+
+		// Update beadID to hook the compound root instead of bare bead
+		beadID = wispRootID
+	}
+
 	// Hook the bead using bd update (discovery-based approach)
 	hookCmd := exec.Command("bd", "update", beadID, "--status=hooked", "--assignee="+targetAgent)
 	hookCmd.Stderr = os.Stderr
@@ -395,6 +458,7 @@ func verifyBeadExists(beadID string) error {
 
 // beadInfo holds status and assignee for a bead.
 type beadInfo struct {
+	Title    string `json:"title"`
 	Status   string `json:"status"`
 	Assignee string `json:"assignee"`
 }