feat(db): implement SqlDialect with MySQL, SQL Server, PostgreSQL dialects

Add three SqlDialect implementations with a factory function:
- MySqlDialect: positional ?, ON DUPLICATE KEY UPDATE, LIMIT/OFFSET
- SqlServerDialect: @pN params, MERGE USING, OFFSET/FETCH
- PostgreSqlDialect: $N (1-based), ON CONFLICT DO UPDATE, LIMIT/OFFSET

TDD approach: 43 tests written first, all passing.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/main/services/database/dialects/index.ts

@@ -0,0 +1,27 @@
+/**
+ * SQL Dialect Factory
+ *
+ * Creates the appropriate SqlDialect implementation based on database type.
+ */
+
+import type { DatabaseType } from '@types/database.types'
+import type { SqlDialect } from '@types/sql-dialect.types'
+
+import { MySqlDialect } from './mysql-dialect'
+import { PostgreSqlDialect } from './postgresql-dialect'
+import { SqlServerDialect } from './sqlserver-dialect'
+
+export { MySqlDialect } from './mysql-dialect'
+export { PostgreSqlDialect } from './postgresql-dialect'
+export { SqlServerDialect } from './sqlserver-dialect'
+
+export function createDialect(type: DatabaseType): SqlDialect {
+  switch (type) {
+    case 'sqlserver':
+      return new SqlServerDialect()
+    case 'postgresql':
+      return new PostgreSqlDialect()
+    default:
+      return new MySqlDialect()
+  }
+}