feat(config): make music base path configurable via env var

hardcoded ~/Music path broke on any non-standard setup.
`music_base_path` on root CLI and `Command` base reads from $MUSIC_BASE_PATH.
`seasons()` and `current_quarter()` now take the path explicitly.
adds .env.example, .envrc, and a pytest covering --help output.

pyproject.toml

@@ -23,6 +23,9 @@ dependencies = [
 music = "music.__main__:cli"
 stream = "music.__main__:stream"
 
+[project.optional-dependencies]
+dev = ["pytest>=8.0"]
+
 [build-system]
 requires = ["hatchling"]
 build-backend = "hatchling.build"