Add skill authoring guidelines and website design audit skill

Establishes standards for creating and organizing AI agent skills in AGENTS.md and provides a reference implementation for performing technical and visual website design audits.

AGENTS.md

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+# AGENTS.md
+
+This file provides guidance to AI coding agents (Claude Code, Cursor, Copilot, etc.) when working with code in this repository.
+
+## Repository Overview
+
+A collection of skills for Claude Code and Claude.ai. Skills are packaged instructions and optional scripts that extend Claude's capabilities.
+
+## Skill Authoring Best Practices
+
+When creating or modifying skills, refer to the official best practices documentation:
+
+https://platform.claude.com/docs/en/agents-and-tools/agent-skills/best-practices
+
+Key principles:
+- Be concise — Claude is already smart, only add context it doesn't have
+- Set appropriate degrees of freedom for the task's fragility
+- Test with the models you plan to use
+
+## Creating a New Skill
+
+### Directory Structure
+
+```
+{skill-name}/               # kebab-case directory name
+  SKILL.md                   # Required: skill definition with YAML frontmatter
+  scripts/                   # Optional: executable scripts
+    {script-name}.sh         # Bash scripts (preferred)
+    {script-name}.py         # Python scripts
+  {supporting-files}.md      # Optional: reference docs, templates, examples
+```
+
+### Naming Conventions
+
+- **Skill directory**: `kebab-case` (e.g., `audit-website-design`, `code-review`)
+- **SKILL.md**: Always uppercase, always this exact filename
+- **Scripts**: `kebab-case.sh` or `kebab-case.py` (e.g., `deploy.sh`, `analyze.py`)
+
+### SKILL.md Format
+
+```markdown
+---
+name: {skill-name}
+description: {One or two sentences describing what the skill does AND when to use it. Include trigger phrases. Max 1024 characters.}
+---
+
+# {Skill Title}
+
+{Brief description of what the skill does.}
+
+## How It Works / Workflow
+
+{Numbered list or sections explaining the skill's workflow}
+
+## Usage
+
+{Show common usage patterns and examples}
+
+## Output
+
+{Describe or show example output}
+```
+
+**Required frontmatter fields:**
+
+| Field | Requirements |
+|-------|-------------|
+| `name` | Max 64 chars, lowercase letters/numbers/hyphens only |
+| `description` | Non-empty, max 1024 chars, third-person voice ("Extracts..." not "I extract...") |
+
+**Optional frontmatter fields:**
+
+| Field | Description |
+|-------|-------------|
+| `disable-model-invocation` | `true` to prevent Claude auto-triggering (manual `/name` only) |
+| `user-invocable` | `false` to hide from `/` menu (background knowledge only) |
+| `allowed-tools` | Space-separated tools pre-approved when skill is active |
+| `argument-hint` | Hint shown in autocomplete, e.g. `[url]` or `[filename]` |
+| `effort` | Reasoning effort: `low`, `medium`, `high`, `max` |
+| `context` | Set to `fork` to run in a subagent |
+| `agent` | Subagent type when `context: fork` (e.g., `Explore`, `Plan`) |
+| `model` | Model override when skill is active |
+| `paths` | Glob patterns limiting when skill activates |
+
+### Best Practices for Context Efficiency
+
+Skills are loaded on-demand — only the skill name and description are loaded at startup. The full `SKILL.md` loads into context only when the agent decides the skill is relevant. To minimize context usage:
+
+- **Keep SKILL.md under 500 lines** — put detailed reference material in separate files
+- **Write specific descriptions** — helps the agent know exactly when to activate the skill
+- **Use progressive disclosure** — reference supporting files that get read only when needed
+- **Prefer scripts over inline code** — script execution doesn't consume context (only output does)
+- **File references should be one level deep** — link directly from SKILL.md to supporting files, avoid chains of references
+
+### Script Best Practices
+
+- Use `#!/bin/bash` shebang and `set -e` for fail-fast behavior
+- Write status messages to stderr: `echo "Message" >&2`
+- Write machine-readable output (JSON) to stdout where applicable
+- Include a cleanup trap for temp files
+- Handle errors explicitly rather than letting Claude figure them out
+- Document all constants (no "magic numbers")
+
+### End-User Installation
+
+**Claude Code (via skills.sh):**
+```bash
+# Install a specific skill
+skills install {skill-name}
+```
+
+**Claude Code (manual):**
+```bash
+cp -r {skill-name} ~/.claude/skills/
+```
+
+**Claude.ai:**
+Add the skill zip to project knowledge or paste SKILL.md contents into the conversation. If the skill requires network access, add required domains at `claude.ai/settings/capabilities`.