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/006_snapshots_table.go

@@ -0,0 +1,43 @@
+package migrations
+
+import (
+	"database/sql"
+	"fmt"
+)
+
+func MigrateSnapshotsTable(db *sql.DB) error {
+	var tableExists bool
+	err := db.QueryRow(`
+		SELECT COUNT(*) > 0
+		FROM sqlite_master
+		WHERE type='table' AND name='issue_snapshots'
+	`).Scan(&tableExists)
+	if err != nil {
+		return fmt.Errorf("failed to check issue_snapshots table: %w", err)
+	}
+
+	if tableExists {
+		return nil
+	}
+
+	_, err = db.Exec(`
+		CREATE TABLE issue_snapshots (
+			id INTEGER PRIMARY KEY AUTOINCREMENT,
+			issue_id TEXT NOT NULL,
+			snapshot_time DATETIME NOT NULL,
+			compaction_level INTEGER NOT NULL,
+			original_size INTEGER NOT NULL,
+			compressed_size INTEGER NOT NULL,
+			original_content TEXT NOT NULL,
+			archived_events TEXT,
+			FOREIGN KEY (issue_id) REFERENCES issues(id) ON DELETE CASCADE
+		);
+		CREATE INDEX idx_snapshots_issue ON issue_snapshots(issue_id);
+		CREATE INDEX idx_snapshots_level ON issue_snapshots(compaction_level);
+	`)
+	if err != nil {
+		return fmt.Errorf("failed to create issue_snapshots table: %w", err)
+	}
+
+	return nil
+}