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)

e2e/docker-compose.smoke.yml

@@ -0,0 +1,11 @@
+# Minimal compose stack for sand-boxer remote smoke tests (SAND-WP-0002-T10).
+services:
+  smoke:
+    image: docker.io/library/nginx:alpine
+    ports:
+      - "127.0.0.1:18080:80"
+    healthcheck:
+      test: ["CMD", "wget", "-q", "--spider", "http://127.0.0.1:80"]
+      interval: 5s
+      timeout: 3s
+      retries: 3