Add a skill package system where each skill is a directory with a skill.yaml
manifest and prompt markdown files. Skills support /command triggers, scoped
config overrides (temperature, max_tokens, tool filtering), chain dependencies
with cycle-safe resolution, and a finish_skill completion signal.
Includes four built-in skills: explore, brainstorm, write-document, and plan.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
