config: drop the dead oanda.account_id (auto-execution leftover), keep it tolerated

The candle provider needs only oanda.api_token + base_url; account_id belonged to the
auto-execution daemon removed 2026-09-03 (31002cb). Removed from config.json.example and
the unused oanda_account_id() accessor deleted; the key stays in _KNOWN_OANDA so existing
config.json files keep loading. CLAUDE.md env-var list updated. Test locks all three.

Suite: 464 passed.

Claude-Session: https://claude.ai/code/session_01RWV2yLt3YXHHGVYHSQU2EE

config.json.example

@@ -26,7 +26,6 @@
     }
   },
   "oanda": {
-    "account_id": "REPLACE_WITH_ACCOUNT_ID",
     "api_token": "REPLACE_WITH_API_TOKEN",
     "base_url": "https://api-fxpractice.oanda.com"
   },