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>

internal/rpc/protocol.go

@@ -233,6 +233,9 @@ type ListArgs struct {
 
 	// Status exclusion (for default non-closed behavior, GH#788)
 	ExcludeStatus []string `json:"exclude_status,omitempty"`
+
+	// Type exclusion (for hiding internal types like gates, bd-7zka.2)
+	ExcludeTypes []string `json:"exclude_types,omitempty"`
 }
 
 // CountArgs represents arguments for the count operation

internal/rpc/server_issues_epics.go

@@ -1117,6 +1117,13 @@ func (s *Server) handleList(req *Request) Response {
 		}
 	}
 
+	// Type exclusion (for hiding internal types like gates, bd-7zka.2)
+	if len(listArgs.ExcludeTypes) > 0 {
+		for _, t := range listArgs.ExcludeTypes {
+			filter.ExcludeTypes = append(filter.ExcludeTypes, types.IssueType(t))
+		}
+	}
+
 	// Guard against excessive ID lists to avoid SQLite parameter limits
 	const maxIDs = 1000
 	if len(filter.IDs) > maxIDs {