test: Podman compatibility — serve, run_capture, and gateway reachability

[kordless]

Goal

Verify that the gateway and agent spawn chain work correctly under Podman (rootless Linux and Podman machine on macOS).

Test matrix

Step	Docker	Podman
nemesis8 serve starts gateway	✓	?
POST /completion spawns container	✓	?
Spawned container can reach gateway via GATEWAY_URL	✓	?
nemesis-mcp.py tools work inside spawned container	✓	?
Port mapping (-p 4000:4000) works	✓	?

Known differences

• Podman rootless on Linux: host is 10.0.2.2 (slirp4netns), not host.docker.internal
• Podman machine on macOS: host is host.containers.internal
• --add-host=host.docker.internal:host-gateway not available on Podman < 4.x
• Socket path differs (already handled by detect_container_socket() in docker.rs)

Dependency

Requires #19 and #21.