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>

config.toml

@@ -1,12 +1,5 @@
-# Non-secret configuration for bggpipe. Secrets (ANTHROPIC_API_KEY,
-# BGG_PASSWORD, BGG_API_TOKEN) come from env vars only and must never
-# appear here.
-
-# The BGG username isn't a secret (it's public on your profile), so it may
-# live here — but BGG_USERNAME from .env OVERRIDES it, and since upload
-# needs BGG_USERNAME anyway, .env alone is enough. Cloners: change or
-# clear this.
-bgg_username = "ewagoner"
+# Non-secret knobs for bggpipe. Everything account-related — including
+# your BGG username — lives in .env (see .env.example), not here.
 
 photos_dir = "photos"
 data_dir = "data"