feat(kevin-exec): size from target_dollars, propagate price, bracket entries

Kevin signals never placed orders: the executor sized only from
sentiment_context["current_price"] (None for Kevin) so qty=0, and orders
were always built SIMPLE (stop/take pcts ignored).

- TradeSignal gains `current_price`; the bridge now sets it on publish
- risk_manager honors `target_dollars` directly (no strength re-scale) and
  resolves price from current_price then sentiment_context
- executor builds BRACKET orders for LONG entries carrying stop/take pcts;
  EXIT/SELL signals stay SIMPLE (the bridge sets pcts even on exits)

[ci skip]

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

services/kevin_signal_bridge/main.py

@@ -129,6 +129,7 @@ class KevinBridge:
             target_dollars=decision.target_dollars,
             stop_loss_pct=Decimal(str(self.config.kevin_stop_loss_pct)),
             take_profit_pct=Decimal(str(self.config.kevin_take_profit_pct)),
+            current_price=current_price,
         )
 
         # Persist to signals table FIRST so downstream FK