Phase 1: Edge schema consolidation infrastructure (Decision 004)

Add metadata and thread_id columns to dependencies table to support:
- Edge metadata: JSON blob for type-specific data (similarity scores, etc.)
- Thread queries: O(1) conversation threading via thread_id

Changes:
- New migration 020_edge_consolidation.go
- Updated Dependency struct with Metadata and ThreadID fields
- Added new entity types: authored-by, assigned-to, approved-by
- Relaxed DependencyType validation (any non-empty string ≤50 chars)
- Added IsWellKnown() and AffectsReadyWork() methods
- Updated SQL queries to include new columns
- Updated tests for new behavior

This enables HOP knowledge graph requirements and Reddit-style threading.

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

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

internal/storage/sqlite/migrations.go

@@ -36,6 +36,7 @@ var migrationsList = []Migration{
 	{"close_reason_column", migrations.MigrateCloseReasonColumn},
 	{"tombstone_columns", migrations.MigrateTombstoneColumns},
 	{"messaging_fields", migrations.MigrateMessagingFields},
+	{"edge_consolidation", migrations.MigrateEdgeConsolidation},
 }
 
 // MigrationInfo contains metadata about a migration for inspection
@@ -79,6 +80,7 @@ func getMigrationDescription(name string) string {
 		"close_reason_column":          "Adds close_reason column to issues table for storing closure explanations (bd-uyu)",
 		"tombstone_columns":            "Adds tombstone columns (deleted_at, deleted_by, delete_reason, original_type) for inline soft-delete (bd-vw8)",
 		"messaging_fields":             "Adds messaging fields (sender, ephemeral, replies_to, relates_to, duplicate_of, superseded_by) for inter-agent communication (bd-kwro)",
+		"edge_consolidation":           "Adds metadata and thread_id columns to dependencies table for edge schema consolidation (Decision 004)",
 	}
 	
 	if desc, ok := descriptions[name]; ok {