Add pivot retest + engulfing strategy with dual take-profit

New strategy (pivot_retest_engulfing) that enters long/short trades at
pivot level retests confirmed by SMA 50 alignment and engulfing candle
patterns. Uses ATR-based stop loss with two take-profit levels — at TP1
half the position closes and SL moves to breakeven, at TP2 the rest closes.

- data_engine: add detect_engulfing() for bullish/bearish pattern detection
- backtester: add generate_signals_pivot_retest(), run_backtest_dual_tp(),
  update signal dispatcher and metrics for dual-TP trade format
- order_executor: support signal=-1 (SHORT), attach SL/TP levels
- config: switch to pivot_retest_engulfing with default params
- chart_trades: new mplfinance script to visualize entries on candlesticks
- README: rewrite with full setup guide, project structure, strategy docs
- requirements.txt: make portable (remove conda file:// paths), add mplfinance
- .env.example: add template for secrets

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

config/.env.example

@@ -0,0 +1,6 @@
+OANDA_API_KEY=your-oanda-api-key-here
+OANDA_ACCOUNT_ID=your-account-id-here
+OANDA_ENV=practice
+DASHBOARD_PASSWORD=changeme
+SUPABASE_URL=https://your-project.supabase.co
+SUPABASE_KEY=your-supabase-anon-key-here