Export SortPolicy type and constants

Export SortPolicy from beads package so external tools (like VC)
can use the sort policy constants without importing internal packages.

Added:
- SortPolicy type alias
- SortPolicyHybrid, SortPolicyPriority, SortPolicyOldest constants

This completes the sort policy feature for external consumers.

beads.go

@@ -46,6 +46,8 @@ type (
 	IssueFilter = types.IssueFilter
 	// WorkFilter represents filtering criteria for work queries.
 	WorkFilter = types.WorkFilter
+	// SortPolicy determines how ready work is ordered.
+	SortPolicy = types.SortPolicy
 	// EpicStatus represents the status of an epic issue.
 	EpicStatus = types.EpicStatus
 )
@@ -75,6 +77,13 @@ const (
 	DepDiscoveredFrom = types.DepDiscoveredFrom
 )
 
+// SortPolicy constants
+const (
+	SortPolicyHybrid   = types.SortPolicyHybrid
+	SortPolicyPriority = types.SortPolicyPriority
+	SortPolicyOldest   = types.SortPolicyOldest
+)
+
 // EventType constants
 const (
 	EventCreated           = types.EventCreated