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>

.env.example

@@ -0,0 +1,20 @@
+PORT=4000
+LOG_LEVEL=info
+
+# Rivet Kit engine (self-hosted in docker-compose)
+RIVET_ENDPOINT=http://localhost:6420
+
+# Docker images used for per-user containers
+GITEA_IMAGE=gitea/gitea:1.22
+OPENCODE_IMAGE=ghcr.io/sst/opencode:latest
+
+# Host where spawned containers expose their ports (usually localhost in dev)
+USER_CONTAINER_HOST=127.0.0.1
+
+# Workspace root on the host that gets bind-mounted into per-user containers.
+# Each user gets a subdir: ${USER_DATA_ROOT}/<userId>/{gitea,workspace}
+USER_DATA_ROOT=./.data/users
+
+# Port range allocated to spawned per-user containers
+USER_PORT_RANGE_START=20000
+USER_PORT_RANGE_END=29999