feat(types): add HOP entity tracking types (bd-7pwh)

- Add EntityRef type for structured entity references with URI support
- Add Creator field to Issue for tracking who created work
- Add Validation type and Validations field for proof-of-stake approvals
- Fix RemoveDependency FK violation on external deps (bd-a3sj)
- Include all new fields in content hash computation
- Full test coverage for all new types

🤝 Generated with [Claude Code](https://claude.ai/code)

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

internal/storage/sqlite/dependencies.go

@@ -221,8 +221,13 @@ func (s *SQLiteStorage) RemoveDependency(ctx context.Context, issueID, dependsOn
 			return fmt.Errorf("failed to record event: %w", err)
 		}
 
-		// Mark both issues as dirty for incremental export
-		if err := markIssuesDirtyTx(ctx, tx, []string{issueID, dependsOnID}); err != nil {
+		// Mark issues as dirty for incremental export
+		// For external refs, only mark the source issue (target doesn't exist locally)
+		issueIDsToMark := []string{issueID}
+		if !strings.HasPrefix(dependsOnID, "external:") {
+			issueIDsToMark = append(issueIDsToMark, dependsOnID)
+		}
+		if err := markIssuesDirtyTx(ctx, tx, issueIDsToMark); err != nil {
 			return wrapDBError("mark issues dirty after removing dependency", err)
 		}