Phase 5: multi-host agents (0.5.0)

- stackpilot-agent: slim token-guarded FastAPI (reuses compose_service) exposing
  stack CRUD/lifecycle/logs + system info; same image, different CMD. agent/
  Dockerfile + compose + .env.example.
- Central proxy: Agent model, agent_service (httpx ping/proxy + live status:
  online/offline/unauthorized + hostname/last_seen), routers/agents.py
  (CRUD + ping + proxied stacks/lifecycle/logs/system).
- Frontend: Settings → Remote hosts (add/check/remove, connectivity dot); Stacks
  grouped by host; remote stack detail with lifecycle, live logs, compose/.env edit.

Verified end-to-end: agent+main on a shared network — register (good/bad token),
list/create/start/logs/delete remote stacks, offline detection (502).

Remote backup destinations (SFTP/S3) deferred.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

agent/Dockerfile

@@ -0,0 +1,14 @@
+# The agent reuses the backend image (same compose/Docker code + deps) and
+# just runs a different ASGI app. Build the backend image first.
+ARG BACKEND_IMAGE=10.10.5.10:3020/menzelj/stackpilot-backend:latest
+FROM ${BACKEND_IMAGE}
+
+ENV STACKS_DIR=/opt/stacks \
+    PORT=5010
+
+EXPOSE 5010
+
+HEALTHCHECK --interval=30s --timeout=5s --start-period=10s \
+    CMD curl -fsS http://localhost:5010/agent/health || exit 1
+
+CMD ["uvicorn", "agent_app:app", "--host", "0.0.0.0", "--port", "5010"]