Remove Tiingo spread monitor (indicative two-tier feed, unvalidatable)

Tiingo's free /tiingo/fx/top publishes only a two-tier indicative
spread for gold ($0.10 liquid hours / $0.70 rollover+news; verified on
273 production samples + live probes) — mid ± a posted increment, not
executable depth, so it can never validate the $0.30 backtest cost
assumption. Removed one day after shipping: tiingo_provider.py, the
sampler/quote plumbing in telegram_bot.py, the channel_spread_detail
config knob and tiingo_api_key resolver, and Module 14's Tiingo tests
(221 -> 210). CLAUDE.md documents the verdict: do not re-add a spread
display from Tiingo's free FX feed.

Claude-Session: https://claude.ai/code/session_01BoZKi67Ct6QTeiFJmybXSw

config.json.example

@@ -3,16 +3,12 @@
     "token": "123456789:REPLACE_WITH_YOUR_BOT_TOKEN",
     "chat_id": "REPLACE_WITH_OWNER_CHAT_ID",
     "channel_id": "-100REPLACE_WITH_CHANNEL_ID_OR_EMPTY_STRING",
-    "alert_min_quality": "C",
-    "channel_spread_detail": false
+    "alert_min_quality": "C"
   },
   "data_providers": {
     "active": "twelve_data",
     "twelve_data": {
       "api_key": "REPLACE_WITH_YOUR_TWELVE_DATA_API_KEY"
-    },
-    "tiingo": {
-      "api_key": ""
     }
   },
   "scanning": {