feat(cerbero): HTTP client with bearer + bot-tag + retry

Client minimale verso Cerbero MCP unified server con header
Authorization Bearer e X-Bot-Tag su ogni richiesta. Retry esponenziale
solo su ConnectionError (non su 4xx). 4xx/5xx -> RuntimeError.

Switch da httpx a requests per allineamento con libreria mock
`responses` (httpx richiederebbe respx). httpx resta in deps per usi
futuri. Aggiunto types-requests ai dev deps per mypy strict.

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

tests/unit/test_cerbero_client.py

@@ -0,0 +1,37 @@
+import pytest
+import responses
+
+from multi_swarm.cerbero.client import CerberoClient
+
+
+@responses.activate
+def test_call_tool_passes_bearer_and_bot_tag() -> None:
+    responses.add(
+        responses.POST,
+        "http://test:9000/mcp-deribit/tools/get_iv_rank",
+        json={"iv_rank": 0.42},
+        status=200,
+    )
+    client = CerberoClient(
+        base_url="http://test:9000", token="tok-xyz", bot_tag="swarm-poc-phase1"
+    )
+    result = client.call_tool("deribit", "get_iv_rank", {"symbol": "BTC-PERPETUAL"})
+    assert result == {"iv_rank": 0.42}
+    req = responses.calls[0].request
+    assert req.headers["Authorization"] == "Bearer tok-xyz"
+    assert req.headers["X-Bot-Tag"] == "swarm-poc-phase1"
+
+
+@responses.activate
+def test_call_tool_raises_on_error() -> None:
+    responses.add(
+        responses.POST,
+        "http://test:9000/mcp-deribit/tools/get_iv_rank",
+        json={"error": "bad"},
+        status=400,
+    )
+    client = CerberoClient(
+        base_url="http://test:9000", token="tok-xyz", bot_tag="swarm-poc-phase1"
+    )
+    with pytest.raises(RuntimeError):
+        client.call_tool("deribit", "get_iv_rank", {})