feat: Implement Step.Gate evaluation (Phase 1: Human Gates)

This implements Phase 1 of the Step.Gate feature (bd-7zka.2):

- bd cook now creates gate issues for steps with gate fields
- Gate issues have type=gate and block the gated step via dependency
- bd list filters out gate issues by default (use --include-gates to show)
- New bd gate command with list and resolve subcommands

Gate types supported in Phase 1:
- human: Manual closure via bd close or bd gate resolve

Implementation details:
- createGateIssue() in cook.go creates gate issues with proper metadata
- collectSteps() creates gate dependencies when processing gated steps
- IssueFilter.ExcludeTypes added to storage layer for type-based filtering
- Gate command provides dedicated UX for gate management

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

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

cmd/bd/list.go

@@ -415,6 +415,9 @@ var listCmd = &cobra.Command{
 		// Template filtering
 		includeTemplates, _ := cmd.Flags().GetBool("include-templates")
 
+		// Gate filtering (bd-7zka.2)
+		includeGates, _ := cmd.Flags().GetBool("include-gates")
+
 		// Parent filtering
 		parentID, _ := cmd.Flags().GetString("parent")
 
@@ -620,6 +623,12 @@ var listCmd = &cobra.Command{
 			filter.IsTemplate = &isTemplate
 		}
 
+		// Gate filtering: exclude gate issues by default (bd-7zka.2)
+		// Use --include-gates or --type gate to show gate issues
+		if !includeGates && issueType != "gate" {
+			filter.ExcludeTypes = append(filter.ExcludeTypes, types.TypeGate)
+		}
+
 		// Parent filtering: filter children by parent issue
 		if parentID != "" {
 			filter.ParentID = &parentID
@@ -721,6 +730,13 @@ var listCmd = &cobra.Command{
 				}
 			}
 
+			// Type exclusion (bd-7zka.2)
+			if len(filter.ExcludeTypes) > 0 {
+				for _, t := range filter.ExcludeTypes {
+					listArgs.ExcludeTypes = append(listArgs.ExcludeTypes, string(t))
+				}
+			}
+
 			resp, err := daemonClient.List(listArgs)
 			if err != nil {
 				fmt.Fprintf(os.Stderr, "Error: %v\n", err)
@@ -961,6 +977,9 @@ func init() {
 	// Template filtering: exclude templates by default
 	listCmd.Flags().Bool("include-templates", false, "Include template molecules in output")
 
+	// Gate filtering: exclude gate issues by default (bd-7zka.2)
+	listCmd.Flags().Bool("include-gates", false, "Include gate issues in output (normally hidden)")
+
 	// Parent filtering: filter children by parent issue
 	listCmd.Flags().String("parent", "", "Filter by parent issue ID (shows children of specified issue)")