fix: inject MCP gateway connection into Codex config.toml

The compiled config.toml had backend MCP server definitions
(container=, guard-policies, entrypointArgs) in mcpg gateway format
that Codex doesn't understand. Codex expects 'command'+'args' (stdio)
or 'url' (HTTP/SSE) fields. This caused dynamic_tool_count=0 at
startup — Codex had zero MCP tools available.

Fix: Before running Codex, rewrite config.toml to replace the backend
server definitions with a single gateway SSE entry pointing to the
already-running MCP gateway at host.docker.internal:$MCP_GATEWAY_PORT.
This gives Codex access to all 31 aggregated tools (playwright,
safeoutputs, etc.) through the gateway.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: lpcox