fix(bun): support extra generated model fields

pkg/writers/bun/README.md

@@ -65,6 +65,11 @@ relspec convert --from pgsql --from-conn "postgres://localhost/mydb" \
 # Multi-file output (one file per table)
 relspec convert --from json --from-path schema.json \
   --to bun --to-path models/ --package models
+
+# Inject computed/scan-only fields that are not present in the database schema
+relspec convert --from dbml --from-path schema.dbml \
+  --to bun --to-path models.go --package models \
+  --extra-fields '[{"target_table":"projects","name":"ThoughtCount","type":"sql_types.SqlInt64","bun_tag":"thought_count,scanonly","json_tag":"thought_count"}]'
 ```
 
 ## Generated Code Examples
@@ -182,6 +187,43 @@ options := &writers.WriterOptions{
 }
 ```
 
+#### Extra fields
+
+Use `Metadata["extra_fields"]` (or the CLI `--extra-fields` flag) to inject
+custom fields into generated Bun models without editing generated files. This is
+intended for computed columns and scan-only query fields that Bun must scan into
+but that are not real database table columns.
+
+Each entry supports:
+
+| Field | Required | Description |
+|---|---:|---|
+| `target_table` | No | Optional model scope. Accepts table name (`projects`), qualified table (`public.projects`), or model name (`ModelPublicProjects`). If omitted, the field is added to every generated model. |
+| `name` | Yes | Go struct field name. |
+| `type` | Yes | Go type to emit. |
+| `bun_tag` | No | Raw Bun tag contents, e.g. `thought_count,scanonly`. |
+| `json_tag` | No | Raw JSON tag contents. |
+| `comment` | No | Optional line comment. |
+
+```go
+options := &writers.WriterOptions{
+    OutputPath:  "models.go",
+    PackageName: "models",
+    Metadata: map[string]any{
+        "extra_fields": `[
+          {
+            "target_table": "projects",
+            "name": "ThoughtCount",
+            "type": "sql_types.SqlInt64",
+            "bun_tag": "thought_count,scanonly",
+            "json_tag": "thought_count",
+            "comment": "Computed by ResolveSpec queries"
+          }
+        ]`,
+    },
+}
+```
+
 ## Notes
 
 - Model names are derived from table names (singularized, PascalCase)