Remove executor, add buy/sell to Alpaca, move capital allocation to Bot

Replace the Executor logging placeholder with real buy/sell methods on
the Alpaca class that place market orders via createOrder and return the
fill price. Strategies now receive their capital amount directly and
place orders themselves. Bot accepts StrategyAllocation[] to decouple
capital allocation from strategy definition.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/alpaca.ts

@@ -36,6 +36,15 @@ export interface AlpacaQuote {
     BidPrice: number;
 }
 
+export interface AlpacaOrder {
+    id: string;
+    symbol: string;
+    filled_avg_price: string;
+    filled_qty: string;
+    side: string;
+    status: string;
+}
+
 export interface AlpacaTrade {
     p: number;  // price
     s: number;  // size
@@ -49,6 +58,13 @@ export interface AlpacaClient {
     getClock(): Promise<AlpacaClock>;
     getLatestQuote(symbol: string): Promise<AlpacaQuote>;
     getLatestTrades(symbols: string[]): Promise<Map<string, AlpacaTrade>>;
+    createOrder(order: {
+        symbol: string;
+        notional: number;
+        side: 'buy' | 'sell';
+        type: string;
+        time_in_force: string;
+    }): Promise<AlpacaOrder>;
 }
 
 export class Alpaca {
@@ -98,5 +114,26 @@ export class Alpaca {
     public async getLatestTrades(symbols: string[]) {
         return this.alpaca.getLatestTrades(symbols);
     }
-    
+
+    public async buy(symbol: string, dollarAmount: number): Promise<number> {
+        const order = await this.alpaca.createOrder({
+            symbol,
+            notional: dollarAmount,
+            side: 'buy',
+            type: 'market',
+            time_in_force: 'day',
+        });
+        return parseFloat(order.filled_avg_price);
+    }
+
+    public async sell(symbol: string, dollarAmount: number): Promise<number> {
+        const order = await this.alpaca.createOrder({
+            symbol,
+            notional: dollarAmount,
+            side: 'sell',
+            type: 'market',
+            time_in_force: 'day',
+        });
+        return parseFloat(order.filled_avg_price);
+    }
 }