Flows MCP server: add a tool to list recent flow runs and their status

We connect Claude Code to the Spacelift Flows MCP server to inspect and edit our flows. Today the MCP tools can list and read blocks, but there is no tool to list recent runs (invocations) of a flow with their status, trigger, and failing block.

When a flow silently stops producing output we have to open the canvas and click through invocations by hand. A list_flow_runs tool (flow ID, time window, status filter) returning run ID, trigger, start time, status, and the first failing block would let an agent diagnose it directly.