fix: remove stale "coming soon" MCP reference in agent-builder prereqs

Line 19 still referenced "Phase 5 MCP server (coming soon)" —
replaced with link to shipped MCP Server Reference docs.

Addresses QA test #10 on ISS-69.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: chuongld20

docs/guides/agent-builder.md

@@ -16,7 +16,7 @@ AI coding agents need the same things human developers need: a machine to run co
 
 - devbox installed and configured ([Quick Start](../getting-started/quickstart.md))
 - At least one server in the pool
-- For MCP: Phase 5 MCP server (coming soon)
+- For MCP: devbox MCP server ([reference](../reference/mcp-server.md))
 
 ## Architecture