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/001_dirty_issues_table.go

@@ -0,0 +1,52 @@
+package migrations
+
+import (
+	"database/sql"
+	"fmt"
+)
+
+func MigrateDirtyIssuesTable(db *sql.DB) error {
+	var tableName string
+	err := db.QueryRow(`
+		SELECT name FROM sqlite_master
+		WHERE type='table' AND name='dirty_issues'
+	`).Scan(&tableName)
+
+	if err == sql.ErrNoRows {
+		_, err := db.Exec(`
+			CREATE TABLE dirty_issues (
+				issue_id TEXT PRIMARY KEY,
+				marked_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+				FOREIGN KEY (issue_id) REFERENCES issues(id) ON DELETE CASCADE
+			);
+			CREATE INDEX idx_dirty_issues_marked_at ON dirty_issues(marked_at);
+		`)
+		if err != nil {
+			return fmt.Errorf("failed to create dirty_issues table: %w", err)
+		}
+		return nil
+	}
+
+	if err != nil {
+		return fmt.Errorf("failed to check for dirty_issues table: %w", err)
+	}
+
+	var hasContentHash bool
+	err = db.QueryRow(`
+		SELECT COUNT(*) > 0 FROM pragma_table_info('dirty_issues')
+		WHERE name = 'content_hash'
+	`).Scan(&hasContentHash)
+	
+	if err != nil {
+		return fmt.Errorf("failed to check for content_hash column: %w", err)
+	}
+	
+	if !hasContentHash {
+		_, err = db.Exec(`ALTER TABLE dirty_issues ADD COLUMN content_hash TEXT`)
+		if err != nil {
+			return fmt.Errorf("failed to add content_hash column: %w", err)
+		}
+	}
+
+	return nil
+}