feat: add 'make dev' task with auto-reload to project template

[hartym]

Summary

Add a make dev task to the generated project template that provides auto-reload functionality during development. Uses watchfiles (Python-native, cross-platform) which is already used by HARP core.

Context

When developing HARP proxy applications, developers need to manually restart the server after each code or configuration change. A make dev task with file watching would significantly improve the development experience.

The cookiecutter template lives at harp/commandline/cookiecutters/project/{{cookiecutter.__dir_name}}/Makefile.

Proposed Implementation

Makefile addition

dev: install  ## Start HARP proxy with auto-reload on file changes
	$(call execute,$(UV) run watchfiles "$(UV) run harp-proxy server{% if cookiecutter.create_application %} --enable {{cookiecutter.__pkg_name}}{% endif %}{% if cookiecutter.create_config %} --file config.yml{% endif %} $(HARP_OPTIONS)"{% if cookiecutter.create_application %} {{cookiecutter.__pkg_name}}/{% endif %}{% if cookiecutter.create_config %} config.yml{% endif %})

pyproject.toml addition

Add watchfiles as a dev dependency:

[project.optional-dependencies]
dev = ["watchfiles"]

Features

• File watching: Monitors config.yml (if enabled) and all files in the application directory (if enabled)
• Cross-platform: Works on macOS, Linux, and Windows without external dependencies
• Proven: Same approach used by HARP core development (harp/commandline/utils/manager.py)

Testing Scenarios

1. Config-only project: Create project with create_application=false, create_config=true. make dev watches only config.yml
2. App project: Create project with create_application=true. make dev watches the app directory
3. Full project: Both config and app enabled, both are watched
4. File change detection: Modify a .py file or config.yml, server should restart automatically