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/007_compaction_config.go

@@ -0,0 +1,26 @@
+package migrations
+
+import (
+	"database/sql"
+	"fmt"
+)
+
+func MigrateCompactionConfig(db *sql.DB) error {
+	_, err := db.Exec(`
+		INSERT OR IGNORE INTO config (key, value) VALUES
+			('compaction_enabled', 'false'),
+			('compact_tier1_days', '30'),
+			('compact_tier1_dep_levels', '2'),
+			('compact_tier2_days', '90'),
+			('compact_tier2_dep_levels', '5'),
+			('compact_tier2_commits', '100'),
+			('compact_model', 'claude-3-5-haiku-20241022'),
+			('compact_batch_size', '50'),
+			('compact_parallel_workers', '5'),
+			('auto_compact_enabled', 'false')
+	`)
+	if err != nil {
+		return fmt.Errorf("failed to add compaction config defaults: %w", err)
+	}
+	return nil
+}