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>

shared/schemas/trading.py

@@ -125,6 +125,7 @@ class TradeSignal(BaseModel):
     target_dollars: Decimal | None = None
     stop_loss_pct: Decimal | None = None
     take_profit_pct: Decimal | None = None
+    current_price: Decimal | None = None
 
     model_config = {"from_attributes": True}