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/types/types.go

@@ -66,6 +66,10 @@ type Issue struct {
 	AwaitID   string        `json:"await_id,omitempty"`   // Condition identifier (e.g., run ID, PR number)
 	Timeout   time.Duration `json:"timeout,omitempty"`    // Max wait time before escalation
 	Waiters   []string      `json:"waiters,omitempty"`    // Mail addresses to notify when gate clears
+
+	// Source tracing fields (gt-8tmz.18): track where this issue came from during cooking
+	SourceFormula   string `json:"source_formula,omitempty"`   // Formula name where this step was defined
+	SourceLocation  string `json:"source_location,omitempty"`  // Path within source: "steps[0]", "advice[0].after"
 }
 
 // ComputeContentHash creates a deterministic hash of the issue's content.