dev-docs: add VS Code debugging and running tests sections

docs/dev-docs.md

@@ -27,6 +27,47 @@ Now link the plugin to your QGIS profile python:
     mklink /J <path-to-QGIS-user-folder>\QGIS3\profiles\default\python\plugins\Mergin <path-to-mergin-qgis-plugin>\Mergin
 ```
 
+### Running tests
+
+Tests live in [`tests/`](../tests) and run via `pytest`. Because the suite depends on QGIS alongside Python testing tooling, conda is the most reliable way to assemble the whole stack. It also enables pinning a specific QGIS version, which is how CI tests across multiple releases. The repo ships an [`environment.yml`](../environment.yml) for it.
+
+1. Install [Miniconda](https://docs.anaconda.com/miniconda/) if you don't have it yet.
+2. Create the test environment :
+   ```
+   conda env create -f environment.yml
+   ```
+3. Activate it and run the suite from the repo root:
+   ```
+   conda activate qgis-test-env
+   pytest ./tests --cov=Mergin
+   ```
+
+## Debugging (VS Code)
+
+1. Install the `python3-debugpy` package. QGIS DevTools relies on `debugpy` being available in QGIS's bundled Python.
+2. Install the [QGIS DevTools](https://github.com/nextgis/qgis_devtools) plugin in QGIS and restart QGIS.
+3. Activate the QGIS DevTools plugin and copy the JSON snippet it provides into a `launch.json` file inside the `.vscode/` folder of this repository:
+
+   ```json
+   {
+       "version": "0.2.0",
+       "configurations": [
+           {
+               "name": "Attach to QGIS",
+               "type": "debugpy",
+               "request": "attach",
+               "connect": {
+                   "host": "127.0.0.1",
+                   "port": 5678
+               },
+               "justMyCode": true
+           }
+       ]
+   }
+   ```
+
+4. Start the debugger.
+
 ## Production
 
 Plugin packages are built as GitHub actions for every commit.