SKILL.md system & skill authoring

The SKILL.md system is a structured way to package specialized knowledge, workflows, and executable code into reusable units that Claude can discover and invoke automatically. Each skill lives in its own directory containing at minimum a SKILL.md file, which opens with YAML frontmatter declaring a name and description, followed by markdown instructions, reference material, and optionally executable scripts or supporting files. Skills are loaded lazily: Claude evaluates each skill's frontmatter description against the current task and only injects the full skill content into its context window when a semantic match occurs. This means a project can include dozens of skills without bloating the active context — only the relevant skill is loaded for any given request. Skills can also include dynamic context injection, where lines like `!\`git diff HEAD\`` are executed before Claude sees the skill content, so real-time data (a current diff, an open GitHub issue) arrives already inlined. In Claude Code, skills are discovered automatically from `~/.claude/skills/` (global) or `.claude/skills/` (project-local). In Claude.ai web and Team/Enterprise plans, skills are uploaded as ZIP archives through the UI or Organization settings. On the API, skills are uploaded programmatically and integrated through the code execution tool. Across all surfaces, skills share the same SKILL.md file format and the same YAML frontmatter schema.