fix(bun): read extra fields from file

pkg/writers/bun/README.md

@@ -67,9 +67,10 @@ 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
+# (extra-fields.json contains the JSON array shown below)
 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"}]'
+  --extra-fields extra-fields.json
 ```
 
 ## Generated Code Examples
@@ -189,11 +190,14 @@ options := &writers.WriterOptions{
 
 #### Extra fields
 
-Use `Metadata["extra_fields"]` (or the CLI `--extra-fields` flag) to inject
+Use `Metadata["extra_fields"]` (or the CLI `--extra-fields <path>` 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.
 
+The CLI flag expects a path to a JSON file, not inline JSON. This avoids shell
+quoting problems with struct tags and complex type names.
+
 Each entry supports:
 
 | Field | Required | Description |
@@ -224,6 +228,21 @@ options := &writers.WriterOptions{
 }
 ```
 
+Example `extra-fields.json`:
+
+```json
+[
+  {
+    "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)