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/014_child_counters_table.go

@@ -0,0 +1,34 @@
+package migrations
+
+import (
+	"database/sql"
+	"fmt"
+)
+
+func MigrateChildCountersTable(db *sql.DB) error {
+	var tableName string
+	err := db.QueryRow(`
+		SELECT name FROM sqlite_master
+		WHERE type='table' AND name='child_counters'
+	`).Scan(&tableName)
+
+	if err == sql.ErrNoRows {
+		_, err := db.Exec(`
+			CREATE TABLE child_counters (
+				parent_id TEXT PRIMARY KEY,
+				last_child INTEGER NOT NULL DEFAULT 0,
+				FOREIGN KEY (parent_id) REFERENCES issues(id) ON DELETE CASCADE
+			)
+		`)
+		if err != nil {
+			return fmt.Errorf("failed to create child_counters table: %w", err)
+		}
+		return nil
+	}
+
+	if err != nil {
+		return fmt.Errorf("failed to check for child_counters table: %w", err)
+	}
+
+	return nil
+}