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/types/types.go

@@ -791,6 +791,9 @@ type IssueFilter struct {
 
 	// Status exclusion (for default non-closed behavior)
 	ExcludeStatus []Status // Exclude issues with these statuses
+
+	// Type exclusion (for hiding internal types like gates)
+	ExcludeTypes []IssueType // Exclude issues with these types
 }
 
 // SortPolicy determines how ready work is ordered