feat: add Claude Code skill for AI assistance

- Add contrib/claude-skill/ with SKILL.md and installation README
- Add docs/content/guide/claude-skill.md for website
- Add `styx @skill` CLI command to output skill content

Users can install with:
  mkdir -p ~/.claude/skills/styx
  styx @Skill > ~/.claude/skills/styx/SKILL.md

Author: fasterthanlime

contrib/claude-skill/README.md

@@ -0,0 +1,39 @@
+# Claude Code Skill for Styx
+
+This skill teaches [Claude Code](https://docs.anthropic.com/en/docs/claude-code) about Styx syntax and schemas.
+
+## Installation
+
+### Option 1: Copy manually
+
+```bash
+mkdir -p ~/.claude/skills/styx
+cp SKILL.md ~/.claude/skills/styx/
+```
+
+### Option 2: Use the CLI
+
+```bash
+mkdir -p ~/.claude/skills/styx
+styx @skill > ~/.claude/skills/styx/SKILL.md
+```
+
+### Option 3: One-liner from repo
+
+```bash
+mkdir -p ~/.claude/skills/styx && curl -sL https://raw.githubusercontent.com/bearcove/styx/main/contrib/claude-skill/SKILL.md > ~/.claude/skills/styx/SKILL.md
+```
+
+## Usage
+
+Once installed, Claude Code will automatically have access to Styx syntax knowledge. You can also explicitly invoke it:
+
+```
+/styx
+```
+
+This teaches Claude about:
+- Styx syntax (scalars, objects, sequences, tags, heredocs)
+- Schema language and type constraints
+- CLI commands for validation and formatting
+- Common mistakes to avoid