feat: add waits-for dependency type for fanout gates (bd-xo1o.2)

Adds 'waits-for' dependency type for dynamic molecule bonding:
- DepWaitsFor blocks an issue until spawner's children are closed
- Two gate types: all-children (wait for all) or any-children (first)
- Updated blocked_cache.go CTE to handle waits-for dependencies
- Added --waits-for and --waits-for-gate flags to bd create command
- Added WaitsForMeta struct for gate metadata storage
- Full test coverage for all gate types and dynamic child scenarios

This enables patrol molecules to wait for dynamically-bonded arms to
complete before proceeding (Christmas Ornament pattern).

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

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

internal/rpc/protocol.go

@@ -72,6 +72,9 @@ type CreateArgs struct {
 	EstimatedMinutes   *int     `json:"estimated_minutes,omitempty"` // Time estimate in minutes
 	Labels             []string `json:"labels,omitempty"`
 	Dependencies       []string `json:"dependencies,omitempty"`
+	// Waits-for dependencies (bd-xo1o.2)
+	WaitsFor     string `json:"waits_for,omitempty"`      // Spawner issue ID to wait for
+	WaitsForGate string `json:"waits_for_gate,omitempty"` // Gate type: all-children or any-children
 	// Messaging fields (bd-kwro)
 	Sender string `json:"sender,omitempty"` // Who sent this (for messages)
 	Wisp   bool   `json:"wisp,omitempty"`   // Wisp = ephemeral vapor from the Steam Engine; bulk-deleted when closed