chore: add Claude Code project context

.claude/skills/docker-swarm-stack.md

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+# Docker Swarm Stack Skill
+
+Use this skill when adding, reviewing, or modifying Docker Swarm stack files in this repository.
+
+## Workflow
+
+1. Confirm the service belongs in the Swarm repo.
+2. Create or update a Swarm-compatible stack YAML file.
+3. Create or update `.env.example` with placeholders only.
+4. Use Docker secrets/configs for sensitive runtime values.
+5. Define overlay networks and named volumes explicitly.
+6. Add `deploy:` settings only when they are meaningful.
+7. Validate YAML/Compose rendering where possible.
+8. Explain deployment commands, required secrets, required networks, and rollback notes.
+
+## Swarm Standards
+
+- Use 2-space YAML indentation.
+- Prefer explicit image tags over unpinned `latest` when stability matters.
+- Use `deploy.restart_policy` for Swarm-managed services.
+- Use `placement.constraints` when a service must run on a specific node or node label.
+- Do not use `container_name`; it is ignored or problematic in Swarm.
+- Be careful with `depends_on`; Swarm does not honor Compose startup ordering the same way local Compose does.
+- Prefer external secrets for sensitive values.
+
+## Validation Command Pattern
+
+```bash
+docker compose --env-file <service>.env.example -f <stack-file>.yml config
+```
+
+Do not run `docker stack deploy` unless explicitly asked.