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/010_content_hash_column.go

@@ -0,0 +1,94 @@
+package migrations
+
+import (
+	"database/sql"
+	"fmt"
+
+	"github.com/steveyegge/beads/internal/types"
+)
+
+func MigrateContentHashColumn(db *sql.DB) error {
+	var colName string
+	err := db.QueryRow(`
+		SELECT name FROM pragma_table_info('issues')
+		WHERE name = 'content_hash'
+	`).Scan(&colName)
+
+	if err == sql.ErrNoRows {
+		_, err := db.Exec(`ALTER TABLE issues ADD COLUMN content_hash TEXT`)
+		if err != nil {
+			return fmt.Errorf("failed to add content_hash column: %w", err)
+		}
+
+		_, err = db.Exec(`CREATE INDEX IF NOT EXISTS idx_issues_content_hash ON issues(content_hash)`)
+		if err != nil {
+			return fmt.Errorf("failed to create content_hash index: %w", err)
+		}
+
+		rows, err := db.Query(`
+			SELECT id, title, description, design, acceptance_criteria, notes,
+			       status, priority, issue_type, assignee, external_ref
+			FROM issues
+		`)
+		if err != nil {
+			return fmt.Errorf("failed to query existing issues: %w", err)
+		}
+		defer rows.Close()
+
+		updates := make(map[string]string)
+		for rows.Next() {
+			var issue types.Issue
+			var assignee sql.NullString
+			var externalRef sql.NullString
+			err := rows.Scan(
+				&issue.ID, &issue.Title, &issue.Description, &issue.Design,
+				&issue.AcceptanceCriteria, &issue.Notes, &issue.Status,
+				&issue.Priority, &issue.IssueType, &assignee, &externalRef,
+			)
+			if err != nil {
+				return fmt.Errorf("failed to scan issue: %w", err)
+			}
+			if assignee.Valid {
+				issue.Assignee = assignee.String
+			}
+			if externalRef.Valid {
+				issue.ExternalRef = &externalRef.String
+			}
+
+			updates[issue.ID] = issue.ComputeContentHash()
+		}
+		if err := rows.Err(); err != nil {
+			return fmt.Errorf("error iterating issues: %w", err)
+		}
+
+		tx, err := db.Begin()
+		if err != nil {
+			return fmt.Errorf("failed to begin transaction: %w", err)
+		}
+		defer tx.Rollback()
+
+		stmt, err := tx.Prepare(`UPDATE issues SET content_hash = ? WHERE id = ?`)
+		if err != nil {
+			return fmt.Errorf("failed to prepare update statement: %w", err)
+		}
+		defer stmt.Close()
+
+		for id, hash := range updates {
+			if _, err := stmt.Exec(hash, id); err != nil {
+				return fmt.Errorf("failed to update content_hash for issue %s: %w", id, err)
+			}
+		}
+
+		if err := tx.Commit(); err != nil {
+			return fmt.Errorf("failed to commit transaction: %w", err)
+		}
+
+		return nil
+	}
+
+	if err != nil {
+		return fmt.Errorf("failed to check content_hash column: %w", err)
+	}
+
+	return nil
+}