feat(capabilities): add write router factory and MCP composition (HUB-WP-0002)

Add create_capability_request_write_router with host workflow callbacks,
CapabilityRequestReroute schema, HubCoreMCPServer.attach_to() with CORE_TOOL_NAMES
exclude filtering, tests, and mark HUB-WP-0002 finished.

tests/test_mcp.py

@@ -1,6 +1,8 @@
 import asyncio
 
-from hub_core.mcp import HubCoreMCPServer
+from fastmcp import FastMCP
+
+from hub_core.mcp import CORE_TOOL_NAMES, HubCoreMCPServer
 
 
 def test_mcp_base_server_constructs_without_registering_tools() -> None:
@@ -29,3 +31,21 @@ def test_mcp_base_server_registers_orientation_doi_and_fos10_tools() -> None:
         "get_risks",
         "get_alerts",
     } <= names
+    assert names == CORE_TOOL_NAMES
+
+
+def test_attach_to_host_mcp_respects_exclude() -> None:
+    host = FastMCP(name="host-hub")
+    HubCoreMCPServer(
+        name="host-hub",
+        api_base="http://127.0.0.1:9999",
+        register_tools=False,
+    ).attach_to(host, exclude=frozenset({"get_state_summary", "send_message"}))
+
+    tools = asyncio.run(host.list_tools())
+    names = {tool.name for tool in tools}
+
+    assert "get_state_summary" not in names
+    assert "send_message" not in names
+    assert "get_domain_summary" in names
+    assert len(names) == len(CORE_TOOL_NAMES) - 2