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_config.py

@@ -13,19 +13,19 @@ def test_defaults_when_no_file(tmp_path, monkeypatch):
 def test_reads_toml(tmp_path, monkeypatch):
     monkeypatch.delenv("BGG_USERNAME", raising=False)
     p = tmp_path / "config.toml"
-    p.write_text(
-        'bgg_username = "someone"\ndata_dir = "elsewhere"\nrate_limit_seconds = 3\n'
-    )
+    p.write_text('data_dir = "elsewhere"\nrate_limit_seconds = 3\n')
     cfg = load_config(p)
-    assert cfg.bgg_username == "someone"
     assert cfg.data_dir == Path("elsewhere")
     assert cfg.rate_limit_seconds == 3.0
     assert cfg.matches_path == Path("elsewhere/matches.csv")
 
 
-def test_env_overrides_toml(tmp_path, monkeypatch):
+def test_username_comes_from_env_only(tmp_path, monkeypatch):
+    # the account has exactly one home: BGG_USERNAME in the environment —
+    # a bgg_username key in config.toml is deliberately ignored
     p = tmp_path / "config.toml"
     p.write_text('bgg_username = "from_toml"\n')
     monkeypatch.setenv("BGG_USERNAME", "from_env")
-    cfg = load_config(p)
-    assert cfg.bgg_username == "from_env"
+    assert load_config(p).bgg_username == "from_env"
+    monkeypatch.delenv("BGG_USERNAME")
+    assert load_config(p).bgg_username == ""

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")