[rushd][WS6] Web UX & MCP bridge #5901

Optional additional clients over the same protocol — a web dashboard bridge and an MCP client — validating the client-agnostic thesis with non-terminal consumers.

Depends on: #5896; #5897

Scope

• @rushstack/rush-daemon-web bridge. A rush daemon serve bridge translating socket⇄WS/HTTP (reusing rush-serve-plugin infra), loopback-only with token auth + an Origin allow-list, and N-tab fan-out from one daemon.
• MCP client. Speak the same daemon protocol (may refit apps/rush-mcp-server), validating the client-agnostic thesis with a non-terminal consumer.

Acceptance criteria

• rush daemon serve bridges the daemon socket to WS/HTTP and fans out to N tabs from one daemon; it binds loopback-only and enforces token auth + an Origin allow-list (non-loopback/bad-origin refused).
• A representative MCP tool call drives a build via the daemon and streams structured results back, consuming only the presentation-free protocol (no terminal assumptions).
• Auth/origin, fan-out, and the MCP path are covered by integration tests; tests green in CI.

Part of #5894.