feat: Add source tracing metadata to cooked steps (gt-8tmz.18)

Add SourceFormula and SourceLocation fields to track where each step
came from during the cooking process. This enables debugging of complex
compositions with inheritance, expansion, and advice.

Changes:
- Added SourceFormula and SourceLocation fields to Step struct (formula/types.go)
- Added same fields to Issue struct (types/types.go)
- Added SetSourceInfo() to parser.go - sets source on all steps after parsing
- Updated cook.go to copy source fields from Step to Issue
- Updated dry-run output to display source info: [from: formula@location]
- Updated advice.go to set source on advice-generated steps
- Updated controlflow.go to preserve source on loop-expanded steps
- Updated expand.go to preserve source on template-expanded steps

The source location format is:
- steps[N] - regular step at index N
- steps[N].children[M] - child step
- steps[N].loop.body[M] - loop body step
- template[N] - expansion template step
- advice - step inserted by advice transformation

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

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

cmd/bd/cook.go

@@ -374,16 +374,18 @@ func collectStepsRecursive(steps []*formula.Step, parentID string, idMapping map
 		}
 
 		issue := &types.Issue{
-			ID:          issueID,
-			Title:       step.Title, // Keep {{variables}} for substitution at pour time
-			Description: step.Description,
-			Status:      types.StatusOpen,
-			Priority:    priority,
-			IssueType:   issueType,
-			Assignee:    step.Assignee,
-			IsTemplate:  true,
-			CreatedAt:   time.Now(),
-			UpdatedAt:   time.Now(),
+			ID:             issueID,
+			Title:          step.Title, // Keep {{variables}} for substitution at pour time
+			Description:    step.Description,
+			Status:         types.StatusOpen,
+			Priority:       priority,
+			IssueType:      issueType,
+			Assignee:       step.Assignee,
+			IsTemplate:     true,
+			CreatedAt:      time.Now(),
+			UpdatedAt:      time.Now(),
+			SourceFormula:  step.SourceFormula,  // Source tracing (gt-8tmz.18)
+			SourceLocation: step.SourceLocation, // Source tracing (gt-8tmz.18)
 		}
 		*issues = append(*issues, issue)
 
@@ -502,7 +504,13 @@ func printFormulaSteps(steps []*formula.Step, indent string) {
 			typeStr = fmt.Sprintf(" (%s)", step.Type)
 		}
 
-		fmt.Printf("%s%s %s: %s%s%s\n", indent, connector, step.ID, step.Title, typeStr, depStr)
+		// Source tracing info (gt-8tmz.18)
+		sourceStr := ""
+		if step.SourceFormula != "" || step.SourceLocation != "" {
+			sourceStr = fmt.Sprintf(" [from: %s@%s]", step.SourceFormula, step.SourceLocation)
+		}
+
+		fmt.Printf("%s%s %s: %s%s%s%s\n", indent, connector, step.ID, step.Title, typeStr, depStr, sourceStr)
 
 		if len(step.Children) > 0 {
 			childIndent := indent