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/012_source_repo_column.go

@@ -0,0 +1,34 @@
+package migrations
+
+import (
+	"database/sql"
+	"fmt"
+)
+
+func MigrateSourceRepoColumn(db *sql.DB) error {
+	var columnExists bool
+	err := db.QueryRow(`
+		SELECT COUNT(*) > 0
+		FROM pragma_table_info('issues')
+		WHERE name = 'source_repo'
+	`).Scan(&columnExists)
+	if err != nil {
+		return fmt.Errorf("failed to check source_repo column: %w", err)
+	}
+
+	if columnExists {
+		return nil
+	}
+
+	_, err = db.Exec(`ALTER TABLE issues ADD COLUMN source_repo TEXT DEFAULT '.'`)
+	if err != nil {
+		return fmt.Errorf("failed to add source_repo column: %w", err)
+	}
+
+	_, err = db.Exec(`CREATE INDEX IF NOT EXISTS idx_issues_source_repo ON issues(source_repo)`)
+	if err != nil {
+		return fmt.Errorf("failed to create source_repo index: %w", err)
+	}
+
+	return nil
+}