Add portainer skill for stack management via API

- 11 scripts for stack lifecycle (start/stop/restart, update, prune)
- Detailed documentation with usage examples and workflow
- Updated README.md files with portainer skill info

README.md

@@ -15,6 +15,7 @@ This repository is intended to be a simple skill source: install the repo (or a
 | Skill | What it does | Path |
 |---|---|---|
 | `gitea-api` | Interact with Gitea via REST API (repos, issues, PRs, releases, branches, user info). | `skills/gitea-api` |
+| `portainer` | Manage Portainer stacks via API (list, start/stop/restart, update, prune images). | `skills/portainer` |
 | `web-automation` | Automate browsing/scraping with Playwright + Camoufox (auth flows, extraction, bot-protected sites). | `skills/web-automation` |
 
 ## Install ideas