Scaffold backend: Hono + Rivet Kit + per-user Docker stack

Backend that provisions per-user OpenCode + Gitea Docker pair via
dockerode and exposes the Grow Agent / sub-agent Rivet Kit actors
described in the PRD. Sub-agent workflows route through the parent
Grow Agent's OpenCode Docker.

- src/docker/manager.ts spawns growqr-gitea-<userId> and growqr-opencode-<userId>
- src/actors/{grow-agent,sub-agent,registry}.ts: Rivet Kit actors
- src/routes/{actors,opencode,git}.ts: PRD section 5.2-5.4 HTTP API
- docker-compose.yml runs rivet-engine + backend (mounts host Docker socket)
- PRD updated to lock in per-user OpenCode/Gitea Docker topology

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitignore

@@ -0,0 +1,9 @@
+node_modules
+dist
+.env
+.env.local
+*.log
+.DS_Store
+
+# Frontend is nested clone — keep it out of backend git for now
+growqr-frontend