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>

Dockerfile

@@ -0,0 +1,20 @@
+FROM node:22-alpine AS base
+WORKDIR /app
+
+FROM base AS deps
+COPY package.json package-lock.json* ./
+RUN npm install
+
+FROM base AS build
+COPY --from=deps /app/node_modules ./node_modules
+COPY tsconfig.json ./
+COPY src ./src
+RUN npx tsc -p tsconfig.json
+
+FROM base AS runtime
+ENV NODE_ENV=production
+COPY --from=deps /app/node_modules ./node_modules
+COPY --from=build /app/dist ./dist
+COPY package.json ./
+EXPOSE 4000
+CMD ["node", "dist/index.js"]