Add portable SKILL.md

[auscaster]

What changed

This PR adds SKILL.md, a portable agent-readable workflow document for nilstate/icey-cli.

Why this belongs in the repo

Agents already inspect README files, package scripts, CI config, Docker docs, and release scripts when working in a repo. A dedicated SKILL.md gives them a stable workflow contract so they can make safer, more repo-specific decisions without changing the runtime, CI, release process, or package layout.

For this repo, the skill focuses on build, validation, packaging, release, Docker demo, and operator bring-up paths.

The goal is not to add another docs page for humans to maintain separately. The goal is to make the repo's existing operational knowledge legible to agents in the same place maintainers already review project conventions.

Why SKILL.md

SKILL.md is becoming a recognizable portable format for agent capabilities and workflow context:

• The Agent Skills specification defines SKILL.md as the required metadata-plus-instructions file for a skill package: https://agentskills.io/specification
• The Agent Skills overview frames skills as portable, version-controlled packages for procedural knowledge and repeatable workflows: https://agentskills.io/
• Anthropic's public skills repository uses self-contained folders with SKILL.md files and links to the Agent Skills specification: https://github.com/anthropics/skills
• Claude's Skills docs describe the progressive-disclosure model: agents load lightweight metadata first, then read the full SKILL.md only when the task matches: https://claude.com/docs/skills/overview

That pattern fits this repo well: icey-server has concrete build, browser-smoke, Docker, release, and package-manager workflows that agents should follow precisely instead of rediscovering from scattered files each time.

Evidence used

• .github/workflows/ci.yml
• .github/workflows/publish-package-managers.yml
• .github/workflows/release.yml
• CMakeLists.txt
• CMakePresets.json
• ICEY_VERSION
• Makefile
• README.md
• VERSION
• docker/README.md
• packaging/README.md
• scripts/build-apt-repo.sh
• scripts/build-deb.sh
• scripts/build-source-archives.sh
• scripts/package-manager-check.sh
• scripts/package-release.sh
• scripts/publish-apt-repo.sh
• scripts/publish-apt-site.sh

Portability

The file is plain markdown. It is useful as project documentation and as agent context. Tools that understand SKILL.md, including runx, can optionally execute or verify it, but this PR does not add a runx dependency.

What this does not do

• It does not change build behavior.
• It does not add generated code.
• It does not add a service dependency.
• It does not add a runx config file.
• It does not publish anything to a registry.

Maintainer review checklist

• Does the file describe the repo's real workflows accurately?
• Are any commands stale, too broad, or missing important prerequisites?
• Is the optional compatibility note acceptable, or should it be removed?
• Should any workflow guidance be narrower before merge?