Finish SAND-WP-0002: remote smoke, podman-compose, HTTP stub

- T10 smoke passed on CoulombCore (sand-boxer self-deploy, sandbox 4e542c51)
- Add e2e smoke compose, scripts/smoke-compose-e2e.sh, make smoke-remote
- Support SANDBOXER_COMPOSE_CMD for podman-compose hosts
- FastAPI v0 stub at sandboxer.api.app; migration gaps doc
- Mark workplan finished (all 10 tasks done)

tests/test_compose_ssh.py

@@ -0,0 +1,14 @@
+"""Compose command configuration."""
+
+from sandboxer.extensions.compose_ssh import ComposeSSHExtension
+
+
+def test_compose_cmd_from_config() -> None:
+    ext = ComposeSSHExtension({"compose_cmd": "podman-compose"})
+    assert ext._compose_bin() == "podman-compose"
+
+
+def test_compose_cmd_env_override(monkeypatch) -> None:
+    monkeypatch.setenv("SANDBOXER_COMPOSE_CMD", "nerdctl compose")
+    ext = ComposeSSHExtension({"compose_cmd": "docker compose"})
+    assert ext._compose_bin() == "nerdctl compose"