feat: add --type-map to override SQL-to-Go types in bun and gorm writers

Adds WriterOptions.TypeMappings and a repeatable --type-map sqltype=gotype
flag. Defaults are unchanged when no mapping is given. Closes #36 (bun/gorm).

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

README.md

@@ -218,6 +218,23 @@ see [`bun`'s `--array-nullable`](./pkg/writers/bun/README.md#nullablearrays)
 flag for nullable-array handling. The `SqlXxxArray` wrapper types remain
 available in `pkg/sqltypes` and are still used by the `gorm` writer.
 
+#### Custom type mapping
+
+Override the built-in SQL → Go mapping of the `bun` and `gorm` writers with the
+repeatable `--type-map sqltype=gotype` flag:
+
+```bash
+relspec convert --from pgsql --from-conn "$DSN" --to gorm --to-path models.go \
+  --type-map uuid=string --type-map jsonb=json.RawMessage
+```
+
+SQL type names are matched case-insensitively on the base type (modifiers such
+as `(10,2)` are ignored; aliases like `int4` resolve to `integer`). NOT NULL
+columns use the Go type verbatim, nullable columns get a `*` prefix (unless the
+type is already a pointer, slice, map or `any`), and arrays become `[]gotype`.
+Unmapped types keep their defaults. The flag does not add imports: use types
+that need none, or add the import afterwards (e.g. with `goimports`).
+
 ## Contributing
 
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