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.

tests/test_cli_help.py

@@ -0,0 +1,16 @@
+import pytest
+from io import StringIO
+from contextlib import redirect_stdout
+
+from pydantic_settings import CliApp
+
+from music.cli import Cli
+
+
+def test_help_shows_music_dir():
+    buf = StringIO()
+    with pytest.raises(SystemExit):
+        with redirect_stdout(buf):
+            CliApp.run(Cli, cli_args=["--help"])
+    output = buf.getvalue()
+    assert "--music-base-path" in output, f"--music-base-path not found in help output:\n{output}"