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/004_closed_at_constraint.go

@@ -0,0 +1,43 @@
+package migrations
+
+import (
+	"database/sql"
+	"fmt"
+)
+
+func MigrateClosedAtConstraint(db *sql.DB) error {
+	var count int
+	err := db.QueryRow(`
+		SELECT COUNT(*)
+		FROM issues
+		WHERE (CASE WHEN status = 'closed' THEN 1 ELSE 0 END) <>
+		      (CASE WHEN closed_at IS NOT NULL THEN 1 ELSE 0 END)
+	`).Scan(&count)
+	if err != nil {
+		return fmt.Errorf("failed to count inconsistent issues: %w", err)
+	}
+
+	if count == 0 {
+		return nil
+	}
+
+	_, err = db.Exec(`
+		UPDATE issues
+		SET closed_at = NULL
+		WHERE status != 'closed' AND closed_at IS NOT NULL
+	`)
+	if err != nil {
+		return fmt.Errorf("failed to clear closed_at for non-closed issues: %w", err)
+	}
+
+	_, err = db.Exec(`
+		UPDATE issues
+		SET closed_at = COALESCE(updated_at, CURRENT_TIMESTAMP)
+		WHERE status = 'closed' AND closed_at IS NULL
+	`)
+	if err != nil {
+		return fmt.Errorf("failed to set closed_at for closed issues: %w", err)
+	}
+
+	return nil
+}