feat: add refs field for cross-references with relationship types (bd-irah)

- Add new DependencyType constants: until, caused-by, validates
- Add --refs flag to bd show for reverse reference lookups
- Group refs by type with appropriate emojis
- Update tests for new dependency types

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

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

internal/types/types.go

@@ -523,6 +523,11 @@ const (
 
 	// Convoy tracking (non-blocking cross-project references)
 	DepTracks DependencyType = "tracks" // Convoy → issue tracking (non-blocking)
+
+	// Reference types (cross-referencing without blocking)
+	DepUntil     DependencyType = "until"     // Active until target closes (e.g., muted until issue resolved)
+	DepCausedBy  DependencyType = "caused-by" // Triggered by target (audit trail)
+	DepValidates DependencyType = "validates" // Approval/validation relationship
 )
 
 // IsValid checks if the dependency type value is valid.
@@ -538,7 +543,8 @@ func (d DependencyType) IsWellKnown() bool {
 	switch d {
 	case DepBlocks, DepParentChild, DepConditionalBlocks, DepWaitsFor, DepRelated, DepDiscoveredFrom,
 		DepRepliesTo, DepRelatesTo, DepDuplicates, DepSupersedes,
-		DepAuthoredBy, DepAssignedTo, DepApprovedBy, DepTracks:
+		DepAuthoredBy, DepAssignedTo, DepApprovedBy, DepTracks,
+		DepUntil, DepCausedBy, DepValidates:
 		return true
 	}
 	return false

internal/types/types_test.go

@@ -522,6 +522,10 @@ func TestDependencyTypeIsWellKnown(t *testing.T) {
 		{DepAuthoredBy, true},
 		{DepAssignedTo, true},
 		{DepApprovedBy, true},
+		{DepTracks, true},
+		{DepUntil, true},
+		{DepCausedBy, true},
+		{DepValidates, true},
 		{DependencyType("custom-type"), false},
 		{DependencyType("unknown"), false},
 	}
@@ -553,6 +557,10 @@ func TestDependencyTypeAffectsReadyWork(t *testing.T) {
 		{DepAuthoredBy, false},
 		{DepAssignedTo, false},
 		{DepApprovedBy, false},
+		{DepTracks, false},
+		{DepUntil, false},
+		{DepCausedBy, false},
+		{DepValidates, false},
 		{DependencyType("custom-type"), false},
 	}