Wire up the MCP startup client

• Status: Done
• Kind: Feature
• Authors: Jean Mertz git@jeanmertz.com
• Date: 2026-08-27
• Implements: 091

Adds a tagged line channel to spawn_stderr_forwarder and a receiver to StartupSet; await_mcp_servers owns one sink per pending server and drops it when that server's join completes, seeding only from the channel's backlog. Also adds a persistent, always-emitted line reporting optional-server startup failures and the tools they take out, pointing to -v for full stderr.