feat(sqlite): add SQLite writer for converting PostgreSQL schemas

- Implement SQLite DDL writer to convert PostgreSQL schemas to SQLite-compatible SQL statements.
- Include automatic schema flattening, type mapping, auto-increment detection, and function translation.
- Add templates for creating tables, indexes, unique constraints, check constraints, and foreign keys.
- Implement tests for writer functionality and data type mapping.

README.md

@@ -59,6 +59,7 @@ RelSpec can write database schemas to multiple formats:
 
 #### Database DDL
 - [PostgreSQL](pkg/writers/pgsql/README.md) - PostgreSQL DDL (CREATE TABLE, etc.)
+- [SQLite](pkg/writers/sqlite/README.md) - SQLite DDL with automatic schema flattening
 
 #### Schema Formats
 - [DBML](pkg/writers/dbml/README.md) - Database Markup Language
@@ -185,6 +186,10 @@ relspec convert --from pgsql --from-conn "postgres://..." \
 # Convert DBML to PostgreSQL SQL
 relspec convert --from dbml --from-path schema.dbml \
                 --to pgsql --to-path schema.sql
+
+# Convert PostgreSQL database to SQLite (with automatic schema flattening)
+relspec convert --from pgsql --from-conn "postgres://..." \
+                --to sqlite --to-path sqlite_schema.sql
 ```
 
 ### Schema Validation