Migrate to Docker: containerize for docker-server deployment

- Add Dockerfile + cron.js (daily 4pm UTC loop replacing EC2 cron)
- Add infra/docker-compose.yml and deploy-stack.sh for Portainer
- Support DATA_DIR env var in bot.js for persistent history volume
- Support PROMPTS_JSON env var in cron.js (no SSH needed for config)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

infra/.env.example

@@ -0,0 +1,23 @@
+# Copy to infra/.env and fill in values. Never commit infra/.env.
+
+# Docker registry (from docker-server setup)
+REGISTRY=registry.yourdomain.com
+
+# API keys
+ANTHROPIC=sk-ant-...
+KERNEL=...
+
+# Prompt config (full JSON from prompts.json, minified)
+PROMPTS_JSON=[{"name":"example","account":"myaccount","prompt":"...","schedule":{"type":"daily","window":[8,20],"postsPerDay":[1,1]}}]
+
+# Account credentials — uppercase prefix must match "account" field in PROMPTS_JSON
+MYACCOUNT_USER=
+MYACCOUNT_PW=
+MYACCOUNT_EMAIL=
+MYACCOUNT_PHONE=
+
+# Add more accounts as needed:
+# OTHERACCOUNT_USER=
+# OTHERACCOUNT_PW=
+# OTHERACCOUNT_EMAIL=
+# OTHERACCOUNT_PHONE=