refactor: keep style-specific config off the base class via **kwargs

The base AlarmStyle contract carried music_file, forcing SimpleStyle to
accept a parameter it never uses. Move style-specific config to the
subclass: the base __init__ takes only set_led (**kwargs swallows the rest);
each style declares the keyword args it actually uses (BlinkStyle: music_file;
SimpleStyle: none). The runner passes config as keyword args and each style
keeps only what it needs, so the base contract never grows as styles are
added — a future 'talk' style adds talk_file to its own signature, not to the
base. README plugin contract updated.

wecker.py

@@ -110,7 +110,7 @@ def run_alarm(
         music_file = os.environ.get("MUSIC_FILE", DEFAULT_MUSIC_FILE)
 
     setup()
-    style = style_cls(set_led, music_file)
+    style = style_cls(set_led, music_file=music_file)
     logging.info("Alarm clock started.")
     try:
         style.start()  # style kicks off its own tone + initial LED