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 == ""