feat: add test-sleep recipe for cluster testing

A simple recipe using alpine:latest that sleeps for 3 minutes
and exits. Useful for testing cluster SSH connectivity, Docker
setup, container auto-removal, and multi-node orchestration
without pulling large model images.

Usage:
  cargo run serve --recipe test-sleep          # 3 min sleep on all nodes
  cargo run stop --recipe test-sleep           # stop early
  cargo run logs --recipe test-sleep           # see container output

recipes/test-sleep/recipe.toml

@@ -0,0 +1,11 @@
+engine = "vllm"
+name = "Test Sleep 3min"
+docker_image = "alpine:latest"
+model = "noop"
+port = 8000
+
+entrypoint = ["sh", "-c", "'sleep 180'"]
+
+args = []
+
+[env]