Remove EUR/USD support — XAU/USD-only codebase

EUR/USD never had a validated edge (2026-07-11/12 verdict: no config
passed validation on M5/M15/H1 at 1-pip cost; cost-to-volatility root
cause). Removed: EurUsdStrategyConfig, pair_signals/eurusd.py, EUR
gates 6h + weekend/day filters + strategy marker + trade geometry in
signal_aggregator, EUR pair registry/aliases in main.py and
gpu_backtest.py, EUR/USD news pair mappings, test_all.py Module 10.
Generic multi-currency news keyword infra kept (GBP/JPY/EUR keyword
table is not pair code). Full EUR history remains at 7d3fd95.

Suite: 162/162 green (was 187; 25 EUR-specific tests removed).

Claude-Session: https://claude.ai/code/session_015kby7M9JwpYi8NQT2yoxAi

telegram_bot.py

@@ -138,7 +138,7 @@ def _build_scan_buttons() -> List[dict]:
     buttons = []
     for symbol in SCAN_PAIRS:
         slug = symbol.replace("/", "").lower()
-        base = PAIR_INFO[symbol]["display"].split("/")[0]  # XAU, EUR
+        base = PAIR_INFO[symbol]["display"].split("/")[0]  # XAU
         prefix = f"{base} " if multi else ""
         for tf, label in _TF_BUTTON_LABELS:
             buttons.append({