mcpdo: add --load (tool load/concurrency test) using the shared core engine #2744

Follow-up to #2439 / #2743, which shipped --load in the one-shot CLI only. That issue was scoped CLI-first ("consider whether to add it to mcpdo later"), so this one is the "later".

What

Give mcpdo a load mode for one tool against a named connection, producing the same summary as mcp-inspector --cli ... --load: ok, toolErrors, errors, errorRate, throughput, latency min/mean/p50/p90/p99/max, and the top error messages.

Why it should be cheap

The engine is already shared. core/cli/handlers/load-test.ts exports runToolLoad(inspectorClient, args, options), resolveLoadOptions, and the flag parsers. It takes an InspectorClient and has no CLI-specific code. The mcpdo work is mostly:

• add the flags (--load, --load-concurrency, --load-duration, --load-requests, --load-rate) to mcpdo's tools/call command, with the same rejection rules as the CLI (each --load-* requires --load; --load requires a tool call);
• run the load inside the daemon, against the daemon's live connection, and return the LoadSummary to the short-lived mcpdo process, rather than driving many separate daemon round trips from the client;
• document it in clients/mcpdo/README.md.

Open question

A long --load-duration holds one daemon request open for the whole run. Check whether the daemon's request/IPC timeouts allow that, and whether other commands on the same connection can still run while a load is in progress.

Priority score

Severity 2 (missing capability; the one-shot CLI works today) + urgency 3 (wanted this milestone) + 1 (milestoned) = 6 → Medium.