feat(mssql): add MSSQL writer for generating DDL from database schema

- Implement MSSQL writer to generate SQL scripts for creating schemas, tables, and constraints.
- Support for identity columns, indexes, and extended properties.
- Add tests for column definitions, table creation, primary keys, foreign keys, and comments.
- Include testing guide and sample schema for integration tests.

pkg/writers/mssql/README.md

@@ -0,0 +1,130 @@
+# MSSQL Writer
+
+Generates Microsoft SQL Server DDL (Data Definition Language) from database schema models.
+
+## Features
+
+- **DDL Generation**: Generates complete SQL scripts for creating MSSQL schema
+- **Schema Support**: Creates multiple schemas with proper naming
+- **Bracket Notation**: Uses [schema].[table] bracket notation for identifiers
+- **Identity Columns**: Generates IDENTITY(1,1) for auto-increment columns
+- **Constraints**: Generates primary keys, foreign keys, unique, and check constraints
+- **Indexes**: Creates indexes with unique support
+- **Extended Properties**: Uses sp_addextendedproperty for comments
+- **Direct Execution**: Can directly execute DDL on MSSQL database
+- **Schema Flattening**: Optional schema flattening for compatibility
+
+## Features by Phase
+
+1. **Phase 1**: Create schemas
+2. **Phase 2**: Create tables with columns, identity, and defaults
+3. **Phase 3**: Add primary key constraints
+4. **Phase 4**: Create indexes
+5. **Phase 5**: Add unique constraints
+6. **Phase 6**: Add check constraints
+7. **Phase 7**: Add foreign key constraints
+8. **Phase 8**: Add extended properties (comments)
+
+## Type Mappings
+
+| Canonical Type | MSSQL Type |
+|----------------|-----------|
+| int | INT |
+| int64 | BIGINT |
+| int16 | SMALLINT |
+| int8 | TINYINT |
+| bool | BIT |
+| float32 | REAL |
+| float64 | FLOAT |
+| decimal | NUMERIC |
+| string | NVARCHAR(255) |
+| text | NVARCHAR(MAX) |
+| timestamp | DATETIME2 |
+| timestamptz | DATETIMEOFFSET |
+| uuid | UNIQUEIDENTIFIER |
+| bytea | VARBINARY(MAX) |
+| date | DATE |
+| time | TIME |
+
+## Usage
+
+### Generate SQL File
+
+```go
+import "git.warky.dev/wdevs/relspecgo/pkg/writers/mssql"
+import "git.warky.dev/wdevs/relspecgo/pkg/writers"
+
+writer := mssql.NewWriter(&writers.WriterOptions{
+    OutputPath: "schema.sql",
+    FlattenSchema: false,
+})
+
+err := writer.WriteDatabase(db)
+if err != nil {
+    panic(err)
+}
+```
+
+### Direct Database Execution
+
+```go
+writer := mssql.NewWriter(&writers.WriterOptions{
+    OutputPath: "",
+    Metadata: map[string]interface{}{
+        "connection_string": "sqlserver://sa:password@localhost/newdb",
+    },
+})
+
+err := writer.WriteDatabase(db)
+if err != nil {
+    panic(err)
+}
+```
+
+### CLI Usage
+
+Generate SQL file:
+```bash
+relspec convert --from json --from-path schema.json \
+                --to mssql --to-path schema.sql
+```
+
+Execute directly to database:
+```bash
+relspec convert --from json --from-path schema.json \
+                --to mssql \
+                --metadata '{"connection_string":"sqlserver://sa:password@localhost/mydb"}'
+```
+
+## Default Values
+
+The writer supports several default value patterns:
+- Functions: `GETDATE()`, `CURRENT_TIMESTAMP()`
+- Literals: strings wrapped in quotes, numbers, booleans (0/1 for BIT)
+- CAST expressions
+
+## Comments/Extended Properties
+
+Table and column descriptions are stored as MS_Description extended properties:
+
+```sql
+EXEC sp_addextendedproperty
+  @name = 'MS_Description',
+  @value = 'Table description here',
+  @level0type = 'SCHEMA', @level0name = 'dbo',
+  @level1type = 'TABLE', @level1name = 'my_table';
+```
+
+## Testing
+
+Run tests with:
+```bash
+go test ./pkg/writers/mssql/...
+```
+
+## Limitations
+
+- Views are not currently supported in the writer
+- Sequences are not supported (MSSQL uses IDENTITY instead)
+- Partitioning and advanced features are not supported
+- Generated DDL assumes no triggers or computed columns