Breaking change: Pi skills must now be named SKILL.md inside a directory,
matching Codex CLI format. Previously any *.md file was treated as a skill.
Migrate by renaming ~/.pi/agent/skills/foo.md to ~/.pi/agent/skills/foo/SKILL.md
* Add skills system with Claude Code compatibility
* consolidate skills into single module, merge loaders, add <available_skills> XML tags
* add Codex CLI skills compatibility, skip hidden/symlinks