devboxverify README registers the shared checkout's cos.js for V-COS #575

User story

As the operator of the test machine, I want the V-COS setup step to name the eve-verify checkout, so that the nightly's cos MCP serves the commit under test.

Acceptance criteria

• The S3 V-COS step in devboxverify/README.md registers mcp/cos.js from the checkout that the eve-verify service runs.

What I'd notice

nothing

Tasks

• Doc edit.

Found while working barelyworkingcode/devboxWorld#42.