test: end-to-end gateway — serve + POST /completion + MCP callback

[kordless]

Goal

Verify the full round-trip:

1. nemesis8 serve starts the gateway on the host (port 4000)
2. POST /completion spawns a container running nemisis8-entry
3. The spawned agent (claude/codex) can use nemesis-mcp.py tools to call back to the gateway

Test steps

# Start the gateway
nemesis8 serve --port 4000

# In another terminal, fire a prompt that uses nemesis-mcp
curl -X POST http://localhost:4000/completion \
  -H "Content-Type: application/json" \
  -d '{"prompt": "Use the nemesis status tool and tell me what you see"}'

Expected: the agent calls GET /status via nemesis-mcp.py and the response includes gateway uptime/active runs.

Dependency

Requires #19 (GATEWAY_URL injection) to be in place first — without it the container cannot reach the host gateway.