Extract SQLite migrations into separate files (bd-fb95094c.7)

- Created migrations/ subdirectory with 14 individual migration files
- Reduced migrations.go from 680 to 98 lines (orchestration only)
- Updated test imports to use migrations package
- Updated MULTI_REPO_HYDRATION.md documentation
- All tests passing

internal/storage/sqlite/migrations/002_external_ref_column.go

@@ -0,0 +1,43 @@
+package migrations
+
+import (
+	"database/sql"
+	"fmt"
+)
+
+func MigrateExternalRefColumn(db *sql.DB) error {
+	var columnExists bool
+	rows, err := db.Query("PRAGMA table_info(issues)")
+	if err != nil {
+		return fmt.Errorf("failed to check schema: %w", err)
+	}
+	defer func() { _ = rows.Close() }()
+
+	for rows.Next() {
+		var cid int
+		var name, typ string
+		var notnull, pk int
+		var dflt *string
+		err := rows.Scan(&cid, &name, &typ, &notnull, &dflt, &pk)
+		if err != nil {
+			return fmt.Errorf("failed to scan column info: %w", err)
+		}
+		if name == "external_ref" {
+			columnExists = true
+			break
+		}
+	}
+
+	if err := rows.Err(); err != nil {
+		return fmt.Errorf("error reading column info: %w", err)
+	}
+
+	if !columnExists {
+		_, err := db.Exec(`ALTER TABLE issues ADD COLUMN external_ref TEXT`)
+		if err != nil {
+			return fmt.Errorf("failed to add external_ref column: %w", err)
+		}
+	}
+
+	return nil
+}