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/.env.example

@@ -0,0 +1,7 @@
+# Shared secret the central StackPilot must present to manage this host.
+# Generate with:  openssl rand -base64 32
+# Enter the SAME value when adding this host under Settings → Remote hosts.
+AGENT_TOKEN=change-me-to-a-long-random-shared-secret
+
+# Host directory where this host's stack folders live.
+STACKS_HOST_DIR=./data/stacks