feat: add nethserver-ns8-module skill

[stephdl]

Goal

Give coding agents (Claude Code and other AGENTS.md-aware tools) the NethServer 8 module development conventions, so they contribute code that follows project best practices instead of reinventing them. Adds a new skill plus the packaging that makes this repository installable as a Claude Code plugin.

What this PR adds

• skills/nethserver-ns8-module/SKILL.md — a single-file skill covering NS8 module development:

  • architecture and directory layout, image pinning + Renovate rules, authorization model (scopes/roles, selfadm)
  • backend patterns: action/event handlers, agent SDK, journald logging, secret handling, service discovery, systemd pod ordering, backup/restore for MariaDB and PostgreSQL
  • frontend patterns: Vue 2 / Vuex / Carbon, ns8-ui-lib component priority, action call pattern, task progress, i18n
  • follows the existing repo template (single SKILL.md, name + description frontmatter, Use when... Supports: description)

• .claude-plugin/marketplace.json + .claude-plugin/plugin.json — turn the repo into a Claude Code plugin marketplace, so all skills install with /plugin marketplace add NethServer/agents + /plugin install nethserver-skills@nethserver. No version is pinned, so every push to the default branch is picked up by background auto-update — users get skill updates without reinstalling.

• README.md — documents the plugin install, background auto-updates, uninstall (Claude Code only), the seven skills by their /name, and usage: a skill is triggered by its /name from the slash menu, by the fully-qualified nethserver-skills:<name>, or automatically when the agent matches its description.

Notes

• Content of the skill is derived from the AGENTS.md guides maintained in the ns8-imapsync module.
• Documentation and config only — no runtime impact on modules.
• Tested end to end: install, uninstall, reinstall, background reload, and skill invocation (auto, slash menu, and fully-qualified name).

discussion at mattermost

[stephdl]

hello @Stell0 I do not have the permissions to merge here on this repo, could you have a go please

[Stell0]

nope, I don't have permissions. @gsanchietti