#770 Agentic Theme A1: MCP client (tools.mcp.call) to call external MCP servers as durable tools

Roadmap Theme A1 (breadth unlock). New engine tools tools.mcp.call + tools.mcp.list_tools (engine/tools/mcp_client) let a workflow or the run_goal agent invoke any external MCP server tool as a durable, replayable, HITL-gateable Highway step, run in a fresh thread so it is loop-context safe. Adds the mcp SDK to pyproject (uv pip install from pyproject, no lock). Validated live: a workflow called the highway-mcp server list_runs over Streamable HTTP and got the real result (run 55712765). NEXT: per-tenant MCP server registry; wire discovered MCP tools into the run_goal catalog + HITL gating; e2e agent test; then Theme A2 managed-OAuth (Composio/Nango).