Username lives in .env only: drop bgg_username from config.toml

load_config no longer reads a bgg_username toml key — BGG_USERNAME in
the environment is its single home (public, but the account should have
exactly one). Docs, messages, and tests updated to match.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/test_smoke_live.py

@@ -1,7 +1,7 @@
 """The one test allowed to touch the real BGG API. Read-only, skipped by
 default; run with: uv run pytest --run-live -m live
 
-Needs bgg_username in config.toml and BGG_API_TOKEN in the environment
+Needs BGG_USERNAME and BGG_API_TOKEN in the environment
 (register at https://boardgamegeek.com/applications).
 """
 
@@ -20,7 +20,7 @@ from bggpipe.config import load_config
 def test_fetch_own_collection_read_only(tmp_path: Path) -> None:
     cfg = load_config()
     if not cfg.bgg_username:
-        pytest.skip("set bgg_username in config.toml to run the live smoke test")
+        pytest.skip("set BGG_USERNAME to run the live smoke test")
     if not os.environ.get("BGG_API_TOKEN"):
         pytest.skip("set BGG_API_TOKEN to run the live smoke test")