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.

src/mpd_now_playable/cli.py

@@ -1,3 +1,4 @@
+import argparse
 import asyncio
 import sys
 from collections.abc import Iterable
@@ -7,6 +8,7 @@ from rich import print as rich_print
 from .__version__ import __version__
 from .config.load import loadConfig
 from .config.model import Config
+from .launchd import DEFAULT_LABEL, install_launchagent, uninstall_launchagent
 from .mpd.listener import MpdStateListener
 from .song_receiver import (
 	Receiver,
@@ -24,20 +26,63 @@ async def listen(
 
 
 def print_help() -> None:
-	print("Usage: mpd-now-playable [OPTIONS]")
-	print("")
-	print("Options:")
-	print("  -h, --help     Show this help message and exit.")
-	print("  -v, --version  Show version and exit.")
+	parser = build_parser()
+	parser.print_help()
+
+
+def build_parser() -> argparse.ArgumentParser:
+	parser = argparse.ArgumentParser(prog="mpd-now-playable")
+	parser.add_argument(
+		"-v",
+		"--version",
+		action="store_true",
+		help="Show version and exit.",
+	)
+
+	subparsers = parser.add_subparsers(dest="command")
+	install_cmd = subparsers.add_parser(
+		"install-launchagent",
+		help="Install and start a per-user launchd service.",
+	)
+	install_cmd.add_argument(
+		"--label",
+		default=DEFAULT_LABEL,
+		help=f"launchd label to install (default: {DEFAULT_LABEL}).",
+	)
+	install_cmd.add_argument(
+		"--force",
+		action="store_true",
+		help="Replace an existing plist with the same label.",
+	)
+
+	uninstall_cmd = subparsers.add_parser(
+		"uninstall-launchagent",
+		help="Unload and remove a per-user launchd service.",
+	)
+	uninstall_cmd.add_argument(
+		"--label",
+		default=DEFAULT_LABEL,
+		help=f"launchd label to uninstall (default: {DEFAULT_LABEL}).",
+	)
+	return parser
 
 
 def main() -> None:
-	args = set(sys.argv[1:])
-	if "-h" in args or "--help" in args:
-		print_help()
+	parser = build_parser()
+	args = parser.parse_args(sys.argv[1:])
+
+	if args.version:
+		rich_print(f"mpd-now-playable v{__version__}")
 		return
-	if "-v" in args or "--version" in args:
-		print(f"mpd-now-playable v{__version__}")
+
+	if args.command == "install-launchagent":
+		path = install_launchagent(label=args.label, force=args.force)
+		rich_print(f"Installed LaunchAgent at {path}")
+		return
+
+	if args.command == "uninstall-launchagent":
+		path = uninstall_launchagent(label=args.label)
+		rich_print(f"Uninstalled LaunchAgent at {path}")
 		return
 
 	rich_print(f"mpd-now-playable v{__version__}")