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/controlflow.go

@@ -146,18 +146,20 @@ func expandLoopIteration(step *Step, iteration int) ([]*Step, error) {
 		iterID := fmt.Sprintf("%s.iter%d.%s", step.ID, iteration, bodyStep.ID)
 
 		clone := &Step{
-			ID:          iterID,
-			Title:       bodyStep.Title,
-			Description: bodyStep.Description,
-			Type:        bodyStep.Type,
-			Priority:    bodyStep.Priority,
-			Assignee:    bodyStep.Assignee,
-			Condition:   bodyStep.Condition,
-			WaitsFor:    bodyStep.WaitsFor,
-			Expand:      bodyStep.Expand,
-			Gate:        bodyStep.Gate,
-			Loop:        cloneLoopSpec(bodyStep.Loop), // Support nested loops (gt-zn35j)
-			OnComplete:  cloneOnComplete(bodyStep.OnComplete),
+			ID:             iterID,
+			Title:          bodyStep.Title,
+			Description:    bodyStep.Description,
+			Type:           bodyStep.Type,
+			Priority:       bodyStep.Priority,
+			Assignee:       bodyStep.Assignee,
+			Condition:      bodyStep.Condition,
+			WaitsFor:       bodyStep.WaitsFor,
+			Expand:         bodyStep.Expand,
+			Gate:           bodyStep.Gate,
+			Loop:           cloneLoopSpec(bodyStep.Loop), // Support nested loops (gt-zn35j)
+			OnComplete:     cloneOnComplete(bodyStep.OnComplete),
+			SourceFormula:  bodyStep.SourceFormula,                                                     // Preserve source (gt-8tmz.18)
+			SourceLocation: fmt.Sprintf("%s.iter%d", bodyStep.SourceLocation, iteration), // Track iteration
 		}
 
 		// Clone ExpandVars if present