Docs: delete stale USAGE.md and superseded pullback plan

USAGE.md documented a different product: USD/JPY (unsupported — the
code is XAU/USD + EUR/USD), 16 main.py flags that don't exist
(--backtest, --optimize, --auto-session-filter, …; main.py has five),
and backtest.py/metrics.py, which aren't in the repo. Two claims were
actively misleading: it called the session filter "default off" (it is
True by default, 03:00-23:00 not 07-16) and gave regime-min-adx as 30
(real default 25). CLAUDE.md now covers the CLI, bot and backtester
correctly, so this is a delete rather than a rewrite.

PULLBACK_ENTRY_PLAN.txt was a pre-implementation draft whose spec no
longer matches what shipped — it specifies a 0.5-2.5 ATR retrace window
and a detector in engine.py, while XAU_PB ships 0.8-6.0 (the drafted
window rejected 81% of valid pullbacks) with the detector in
signal_aggregator.py so live and backtest share one code path. Its
validated outcome (+38.3R / 254 trades / 12mo M5) and every shipped
gate are recorded in CLAUDE.md and the config docstrings.

Also drops the now-dangling references to both files from CLAUDE.md
and the config.py XAU_PB comment.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

config.py

@@ -251,7 +251,7 @@ class XauStrategyConfig:
     # halves of 6mo and 12mo backtests).
     m5_sell_min_rsi: float = 25.0
 
-    # ── M5 pullback entry (XAU_PB — see PULLBACK_ENTRY_PLAN.txt) ──
+    # ── M5 pullback entry (XAU_PB) ──
     # Second entry archetype: enter on the retrace to EMA21/KC-mid in
     # an established trend instead of at Keltner-breakout extension.
     # Validated 2026-07: +38.3R/254 trades on 12mo, positive both