feat(agents): add bruno section

.agents/DEVELOPER.md

@@ -61,6 +61,72 @@ The default value should match what is defined in `pdfengines.go`'s `fs.StringSl
 - **Mocks:** Comprehensive mock implementations for all major interfaces live in `pkg/gotenberg/mocks.go`. Use these for unit tests.
 - **No business logic in `cmd/`:** The `cmd/gotenberg/` package is strictly for wiring and startup.
 
+## Bruno API Collection
+
+A [Bruno](https://www.usebruno.com/) collection lives in `.bruno/` and mirrors every Gotenberg route. When adding or updating a route, update the collection to match.
+
+### Structure
+
+```
+.bruno/
+├── bruno.json                     # Collection config
+├── collection.bru                 # Collection-level defaults (Gotenberg-Trace header)
+├── environments/
+│   ├── Local.bru                  # baseUrl: http://localhost:3000
+│   └── Demo.bru                   # baseUrl: https://demo.gotenberg.dev
+├── Health & Info/                  # GET routes
+├── Chromium/Convert/               # POST routes grouped by module
+├── Chromium/Screenshot/
+├── LibreOffice/
+└── PDF Engines/<Feature>/          # One folder per feature (Merge, Split, Rotate, …)
+```
+
+### `.bru` file format
+
+```bru
+meta {
+  name: <Human-readable name>
+  type: http
+  seq: <order within folder>
+}
+
+post {
+  url: {{baseUrl}}/forms/<path>
+  body: multipartForm
+  auth: none
+}
+
+body:multipart-form {
+  files: @file(../../test/integration/testdata/<file>)
+  <mandatoryField>: <value>
+  ~<optionalField>: <value>
+}
+
+headers {
+  ~Gotenberg-Output-Filename: <name>
+  ~Gotenberg-Webhook-Url: http://localhost:8080/webhook
+  ~Gotenberg-Webhook-Error-Url: http://localhost:8080/webhook/error
+  ~Gotenberg-Webhook-Method: POST
+  ~Gotenberg-Webhook-Error-Method: POST
+  ~Gotenberg-Webhook-Extra-Http-Headers: {"X-Custom":"value"}
+}
+```
+
+### Conventions
+
+- **Mandatory fields** are listed without prefix; **optional fields** are prefixed with `~` (disabled by default in Bruno).
+- **File references** use relative paths to `test/integration/testdata/`.
+- **Webhook and output filename headers** are included on every POST route as optional (`~`).
+- **One `.bru` file per request**. For routes with read/write variants (e.g., bookmarks, metadata), create separate files in the same folder.
+
+### Checklist when adding/updating a route
+
+1. Create or update the `.bru` file in the matching folder under `.bruno/`.
+2. Include all form fields from the route handler — check `FormData*` calls in the route function.
+3. For file upload fields (`files`, `watermark`, `stamp`, `embeds`), use `@file(...)` with a suitable test file.
+4. Verify the URL path matches the route's `Path` field exactly.
+5. If you add a new module folder, keep the naming consistent (e.g., `PDF Engines/Rotate/`).
+
 ## Documentation
 
 - Do not modify `README.md` unless explicitly asked.