Add support for Agent Skills

[jan-xyz]

https://www.anthropic.com/news/skills

https://agentskills.io/home

[kolaente]

Codex is now implementing support for this as well: https://github.com/openai/codex/blob/main/docs/skills.md

[volfadar]

droid as well https://docs.factory.ai/cli/configuration/skills

[meowgorithm]

Hey everyone! This is being implemented in: #1690.