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/expand.go

@@ -163,12 +163,14 @@ func expandStep(target *Step, template []*Step, depth int) ([]*Step, error) {
 
 	for _, tmpl := range template {
 		expanded := &Step{
-			ID:          substituteTargetPlaceholders(tmpl.ID, target),
-			Title:       substituteTargetPlaceholders(tmpl.Title, target),
-			Description: substituteTargetPlaceholders(tmpl.Description, target),
-			Type:        tmpl.Type,
-			Priority:    tmpl.Priority,
-			Assignee:    tmpl.Assignee,
+			ID:             substituteTargetPlaceholders(tmpl.ID, target),
+			Title:          substituteTargetPlaceholders(tmpl.Title, target),
+			Description:    substituteTargetPlaceholders(tmpl.Description, target),
+			Type:           tmpl.Type,
+			Priority:       tmpl.Priority,
+			Assignee:       tmpl.Assignee,
+			SourceFormula:  tmpl.SourceFormula,  // Preserve source from template (gt-8tmz.18)
+			SourceLocation: tmpl.SourceLocation, // Preserve source location (gt-8tmz.18)
 		}
 
 		// Substitute placeholders in labels