fix: write single first-artist tag, not doubled/multi-artist

Live end-to-end test surfaced two bugs in youtube tagging:
- `--replace-in-metadata artist .* NAME` matched twice and doubled the
  artist tag (e.g. "SLVMLORDSLVMLORD"). Anchor with ^.*$ to match once.
- Use only the first artist when several are present (SLVMLORD, not
  "SLVMLORD, Travis Bradley, ...") for both the embedded tag and the
  spoken/echoed API messages.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/test_actions.py

@@ -28,6 +28,17 @@ def test_done_message_mentions_title():
     assert "Avril Lavigne" in msg
 
 
+def test_messages_use_only_first_artist():
+    hit = mf.Hit(source="youtube", kind="track", title="Under My Skin",
+                 artist="SLVMLORD, James John, BobbyGee", album="X", year="",
+                 payload={"videoId": "abc"})
+    for msg in (actions.started_message(hit), actions.done_message(hit),
+                actions.failed_message(hit)):
+        assert "SLVMLORD" in msg
+        assert "James John" not in msg
+        assert "BobbyGee" not in msg
+
+
 def test_perform_youtube_calls_act_youtube(monkeypatch):
     calls = {}
     monkeypatch.setattr(mf, "act_youtube",