MCP tool search mode: when an agent has >30 MCP tools, tools are
deferred and a single mcp_tool_search meta-tool is registered instead.
Uses BM25 index for keyword search, activating matched tools on demand.
Shared connection pool: introduces Pool that maintains one physical
connection per MCP server name, shared across all agents via refcount.
Eliminates N×M duplicate connections (N agents × M servers).
