Implement fanout gate: waits-for children aggregation (gt-8tmz.38)

- Add children-of(step-id) syntax for explicit spawner reference
- Add WaitsForSpec type and ParseWaitsFor() helper
- Update cook.go to create DepWaitsFor dependencies with metadata
- Infer spawner from first needs entry when using all-children
- Add validation tests for children-of() syntax
- Add unit tests for ParseWaitsFor()

This completes the Christmas Ornament aggregation pattern:
- survey-workers does for-each → creates N children
- aggregate waits-for children-of(survey-workers)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

cmd/bd/cook.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"context"
+	"encoding/json"
 	"fmt"
 	"os"
 	"strings"
@@ -720,6 +721,36 @@ func collectDependencies(step *formula.Step, idMapping map[string]string, deps *
 		})
 	}
 
+	// Process waits_for field (gt-8tmz.38) - fanout gate dependency
+	if step.WaitsFor != "" {
+		waitsForSpec := formula.ParseWaitsFor(step.WaitsFor)
+		if waitsForSpec != nil {
+			// Determine spawner ID
+			spawnerStepID := waitsForSpec.SpawnerID
+			if spawnerStepID == "" && len(step.Needs) > 0 {
+				// Infer spawner from first need
+				spawnerStepID = step.Needs[0]
+			}
+
+			if spawnerStepID != "" {
+				if spawnerIssueID, ok := idMapping[spawnerStepID]; ok {
+					// Create WaitsFor dependency with metadata
+					meta := types.WaitsForMeta{
+						Gate: waitsForSpec.Gate,
+					}
+					metaJSON, _ := json.Marshal(meta)
+
+					*deps = append(*deps, &types.Dependency{
+						IssueID:     issueID,
+						DependsOnID: spawnerIssueID,
+						Type:        types.DepWaitsFor,
+						Metadata:    string(metaJSON),
+					})
+				}
+			}
+		}
+	}
+
 	// Recursively handle children
 	for _, child := range step.Children {
 		collectDependencies(child, idMapping, deps)