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/advice.go

@@ -166,10 +166,13 @@ func adviceStepToStep(as *AdviceStep, target *Step) *Step {
 	desc := substituteStepRef(as.Description, target)
 
 	return &Step{
-		ID:          id,
-		Title:       title,
-		Description: desc,
-		Type:        as.Type,
+		ID:            id,
+		Title:         title,
+		Description:   desc,
+		Type:          as.Type,
+		SourceFormula: target.SourceFormula, // Inherit source formula from target (gt-8tmz.18)
+		// SourceLocation will be "advice" to indicate this came from advice transformation
+		SourceLocation: "advice",
 	}
 }