The three MCP siblings (mcp_client/mcp_tool/mcp_tools_cache) served one
objective but sat loose at the top of shared/tools. Grouped them into an
mcp/ package and dropped the redundant prefix: client.py, tool.py, cache.py.
Updated all importers (routes, mcp_tools subagent, e2e fake patch targets,
unit test) to the new paths.
