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/formula/types.go

@@ -183,7 +183,8 @@ type Step struct {
 	Children []*Step `json:"children,omitempty"`
 
 	// Gate defines an async wait condition for this step.
-	// TODO(bd-7zka): Not yet implemented in bd cook. Will integrate with bd-udsi gates.
+	// When set, bd cook creates a gate issue that blocks this step.
+	// Close the gate issue (bd close bd-xxx.gate-stepid) to unblock.
 	Gate *Gate `json:"gate,omitempty"`
 
 	// Loop defines iteration for this step.
@@ -206,8 +207,9 @@ type Step struct {
 	SourceLocation string `json:"-"` // Internal only, not serialized to JSON
 }
 
-// Gate defines an async wait condition (integrates with bd-udsi).
-// TODO(bd-7zka): Not yet implemented in bd cook. Schema defined for future use.
+// Gate defines an async wait condition for formula steps.
+// When a step has a Gate, bd cook creates a gate issue that blocks the step.
+// The gate must be closed (manually or via watchers) to unblock the step.
 type Gate struct {
 	// Type is the condition type: gh:run, gh:pr, timer, human, mail.
 	Type string `json:"type"`