feat(invoice): implement atomic invoice tools (REFACTOR-3)

- Add create_invoice_draft, add_invoice_line, update_invoice_line, delete_invoice_line, set_invoice_project, validate_invoice tools
- Update DolibarrClient with corresponding methods
- Add tests for atomic invoice operations
- Update development docs with venv instructions

docs/development.md

@@ -6,18 +6,25 @@ kept separate in `docker/`.
 
 ## Install development dependencies
 
+It is recommended to use a virtual environment to avoid conflicts with system packages (especially on Linux systems with externally managed environments).
+
 ```bash
-pip install -e '.[dev]'
-```
+# Create a virtual environment
+python3 -m venv .venv
 
-### Windows PowerShell
+# Activate the virtual environment
+source .venv/bin/activate  # On Linux/macOS
+# .venv\Scripts\activate   # On Windows
 
-```powershell
-pip install -e .`[dev`]
+# Install dependencies
+pip install -r requirements.txt
+pip install -e .
 ```
 
 ## Run the test suite
 
+Once your virtual environment is active:
+
 ```bash
 pytest
 ```
@@ -28,6 +35,12 @@ To gather coverage metrics:
 pytest --cov=src/dolibarr_mcp --cov-report=term-missing
 ```
 
+If you encounter "command not found" errors, ensure your virtual environment is activated or run via python module:
+
+```bash
+python3 -m pytest
+```
+
 ## Formatting and linting
 
 The project intentionally avoids heavy linting dependencies. Follow the coding