feat(GB-51): Add monthly_performance MCP tool

- Add monthly_performance tool with year/month parameters
- Calls /api/trades/monthly/{year}/{month} endpoint
- Returns comprehensive monthly analytics

Enables January 2026 loss analysis via Claude Code.

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

src/server.py

@@ -196,6 +196,18 @@ async def list_tools() -> list[Tool]:
             description="Get recent trades with P/L details",
             inputSchema={"type": "object", "properties": {}, "required": []},
         ),
+        Tool(
+            name="monthly_performance",
+            description="Get monthly trade performance analytics (win rate, P/L, exit reasons, TP/SL used)",
+            inputSchema={
+                "type": "object",
+                "properties": {
+                    "year": {"type": "integer", "description": "Year (e.g., 2026)"},
+                    "month": {"type": "integer", "description": "Month (1-12)"},
+                },
+                "required": ["year", "month"],
+            },
+        ),
         Tool(
             name="expectancy",
             description="Get trading expectancy metrics for all symbols",
@@ -632,6 +644,10 @@ async def call_tool(name: str, arguments: dict[str, Any]) -> list[TextContent]:
             result = await api_get("/api/correlations/top")
         case "trades":
             result = await api_get("/api/dashboard/trades")
+        case "monthly_performance":
+            year = arguments.get("year")
+            month = arguments.get("month")
+            result = await api_get(f"/api/trades/monthly/{year}/{month}")
         case "expectancy":
             result = await api_get("/api/expectancy/")
         case "profitable_symbols":