cli: add first-party launchd install/uninstall commands for autostart

Introduce built-in launchd management to remove shell-wrapper indirection and
make macOS autostart reliable.

- Add `install-launchagent` and `uninstall-launchagent` CLI subcommands.
- Create launchd helper module to:
  - write `~/Library/LaunchAgents/<label>.plist`
  - invoke app via `sys.executable -m mpd_now_playable.cli` (absolute interpreter path)
  - set `RunAtLoad`, `KeepAlive`
  - run `launchctl bootout/bootstrap/kickstart` for install and `bootout` for uninstall
  - support `--label` and `--force`
- Add tests for subcommand dispatch, plist generation, and launchctl flow.
- Document the new launchd workflow in README.
  - Run markdownlint

This keeps the setup DRY/KISS, avoids PATH/shell expansion pitfalls in launchd,
and gives users a supported and easy autostart path out of the box.

tests/test_launchd.py

@@ -0,0 +1,77 @@
+import plistlib
+from pathlib import Path
+
+import pytest
+
+import mpd_now_playable.launchd as launchd
+
+
+def test_plist_bytes_uses_absolute_python(monkeypatch, tmp_path) -> None:
+	monkeypatch.setattr(launchd.sys, "executable", "/abs/python")
+	monkeypatch.setattr(Path, "home", lambda: tmp_path)
+
+	payload = plistlib.loads(launchd._plist_bytes("com.example.test"))
+
+	assert payload["Label"] == "com.example.test"
+	assert payload["ProgramArguments"] == ["/abs/python", "-m", "mpd_now_playable.cli"]
+	assert payload["RunAtLoad"] is True
+	assert payload["KeepAlive"] is True
+
+
+def test_install_launchagent_writes_plist_and_bootstraps(monkeypatch, tmp_path) -> None:
+	monkeypatch.setattr(launchd.sys, "platform", "darwin")
+	monkeypatch.setattr(launchd.sys, "executable", "/abs/python")
+	monkeypatch.setattr(Path, "home", lambda: tmp_path)
+	monkeypatch.setattr(launchd.os, "getuid", lambda: 501)
+
+	seen: list[tuple[tuple[str, ...], bool]] = []
+
+	def fake_run_launchctl(*args: str, check: bool = True) -> object:
+		seen.append((args, check))
+		return object()
+
+	monkeypatch.setattr(launchd, "_run_launchctl", fake_run_launchctl)
+
+	plist_path = launchd.install_launchagent(label="com.example.test", force=False)
+	assert plist_path == tmp_path / "Library" / "LaunchAgents" / "com.example.test.plist"
+	assert plist_path.exists()
+	assert seen == [
+		(("bootout", "gui/501/com.example.test"), False),
+		(("bootstrap", "gui/501", str(plist_path)), True),
+		(("kickstart", "-k", "gui/501/com.example.test"), True),
+	]
+
+
+def test_install_launchagent_refuses_existing_without_force(monkeypatch, tmp_path) -> None:
+	monkeypatch.setattr(launchd.sys, "platform", "darwin")
+	monkeypatch.setattr(Path, "home", lambda: tmp_path)
+
+	plist = tmp_path / "Library" / "LaunchAgents" / "com.example.test.plist"
+	plist.parent.mkdir(parents=True, exist_ok=True)
+	plist.write_text("already here")
+
+	with pytest.raises(FileExistsError):
+		launchd.install_launchagent(label="com.example.test")
+
+
+def test_uninstall_launchagent_boots_out_and_removes_file(monkeypatch, tmp_path) -> None:
+	monkeypatch.setattr(launchd.sys, "platform", "darwin")
+	monkeypatch.setattr(Path, "home", lambda: tmp_path)
+	monkeypatch.setattr(launchd.os, "getuid", lambda: 501)
+
+	seen: list[tuple[tuple[str, ...], bool]] = []
+
+	def fake_run_launchctl(*args: str, check: bool = True) -> object:
+		seen.append((args, check))
+		return object()
+
+	monkeypatch.setattr(launchd, "_run_launchctl", fake_run_launchctl)
+
+	plist = tmp_path / "Library" / "LaunchAgents" / "com.example.test.plist"
+	plist.parent.mkdir(parents=True, exist_ok=True)
+	plist.write_text("x")
+
+	removed = launchd.uninstall_launchagent(label="com.example.test")
+	assert removed == plist
+	assert not plist.exists()
+	assert seen == [(("bootout", "gui/501/com.example.test"), False)]