feat(V2): /mcp-cross/tools/get_historical with cross-exchange consensus

Add a unified historical endpoint that fans out to every exchange
supporting the requested (asset_class, symbol) pair, then merges the
results into a single consensus candle series with per-bar divergence
metrics:
  - candles[i].close = median across sources
  - candles[i].sources = count of contributing exchanges
  - candles[i].div_pct = (max-min)/median for that bar's close

Crypto routes BTC/ETH/SOL across Bybit + Hyperliquid + Deribit; equities
route to Alpaca for now (IBKR omitted from MVP because its bars endpoint
takes a relative period instead of start/end). Partial failures return a
warning envelope (failed_sources) instead of failing the whole request;
all sources failing → HTTP 502.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/cerbero_mcp/exchanges/cross/tools.py

@@ -0,0 +1,28 @@
+"""Pydantic schemas + thin tool wrappers for the /mcp-cross router."""
+from __future__ import annotations
+
+from typing import Literal
+
+from pydantic import BaseModel
+
+from cerbero_mcp.exchanges.cross.client import CrossClient
+
+AssetClass = Literal["crypto", "stocks"]
+
+
+class GetHistoricalReq(BaseModel):
+    symbol: str
+    asset_class: AssetClass = "crypto"
+    interval: str = "1h"
+    start_date: str
+    end_date: str
+
+
+async def get_historical(client: CrossClient, params: GetHistoricalReq) -> dict:
+    return await client.get_historical(
+        symbol=params.symbol,
+        asset_class=params.asset_class,
+        interval=params.interval,
+        start_date=params.start_date,
+        end_date=params.end_date,
+    )