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>

internal/formula/parser.go

@@ -92,6 +92,10 @@ func (p *Parser) ParseFile(path string) (*Formula, error) {
 	}
 
 	formula.Source = absPath
+
+	// Set source tracing info on all steps (gt-8tmz.18)
+	SetSourceInfo(formula)
+
 	p.cache[absPath] = formula
 
 	// Also cache by name for extends resolution
@@ -393,3 +397,30 @@ func ApplyDefaults(formula *Formula, values map[string]string) map[string]string
 
 	return result
 }
+
+// SetSourceInfo sets SourceFormula and SourceLocation on all steps in a formula.
+// Called after parsing to enable source tracing during cooking (gt-8tmz.18).
+func SetSourceInfo(formula *Formula) {
+	setSourceInfoRecursive(formula.Steps, formula.Formula, "steps")
+	// Also set source info on template steps for expansion formulas
+	setSourceInfoRecursive(formula.Template, formula.Formula, "template")
+}
+
+// setSourceInfoRecursive recursively sets source info on steps.
+func setSourceInfoRecursive(steps []*Step, formulaName, pathPrefix string) {
+	for i, step := range steps {
+		step.SourceFormula = formulaName
+		step.SourceLocation = fmt.Sprintf("%s[%d]", pathPrefix, i)
+
+		if len(step.Children) > 0 {
+			childPath := fmt.Sprintf("%s[%d].children", pathPrefix, i)
+			setSourceInfoRecursive(step.Children, formulaName, childPath)
+		}
+
+		// Handle loop body steps
+		if step.Loop != nil && len(step.Loop.Body) > 0 {
+			bodyPath := fmt.Sprintf("%s[%d].loop.body", pathPrefix, i)
+			setSourceInfoRecursive(step.Loop.Body, formulaName, bodyPath)
+		}
+	}
+}