Implement Agent Skills standard (https://agentskills.io/specification):
- Validate name (must match parent dir, lowercase, max 64 chars)
- Validate description (required, max 1024 chars)
- Warn on unknown frontmatter fields
- Warn on name collisions (keep first)
- Change prompt format to XML structure
- Remove {baseDir} placeholder (use relative paths)
- Add tests and update documentation
fixes#231
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