Add skill-authoring skill for creating user-facing skills

Meta-skill documenting how to create and maintain agent skills:
- Skill structure (SKILL.md + optional directories)
- Frontmatter metadata requirements
- Progressive disclosure principles (metadata → instructions → references)
- Token budgets and context efficiency
- Optional directories (scripts/, references/, assets/)
- File reference patterns
- Skill categories (developer vs user-facing)
- Writing effective skills checklist

Reference documentation includes:
- PATTERNS.md: Patterns learned from B2C skills including content
  organization, effective tables, code examples, XML validation,
  progressive complexity, common skill templates, and anti-patterns

Canonical reference: https://agentskills.io/home

Author: clavery

.claude/skills/skill-authoring/SKILL.md

@@ -0,0 +1,228 @@
+---
+name: skill-authoring
+description: Guide for creating and maintaining user-facing agent skills
+---
+
+# Skill Authoring Guide
+
+This skill guides you through creating user-facing agent skills. For the canonical reference, see [agentskills.io](https://agentskills.io/home).
+
+## Skill Structure
+
+A skill is a folder containing a `SKILL.md` file with metadata and instructions:
+
+```
+my-skill/
+├── SKILL.md          # Required: instructions + metadata
+├── scripts/          # Optional: executable code
+├── references/       # Optional: additional documentation
+└── assets/           # Optional: templates, resources
+```
+
+## SKILL.md Format
+
+### Required Frontmatter
+
+```yaml
+---
+name: skill-name
+description: Brief description of what the skill does
+---
+```
+
+- **name**: Unique identifier (lowercase, hyphens)
+- **description**: One-line summary (loaded at startup for all skills)
+
+### Instructions Body
+
+The body contains markdown instructions that tell the agent how to perform the task.
+
+```markdown
+---
+name: my-skill
+description: Does something useful
+---
+
+# Skill Title
+
+Brief overview of what this skill helps accomplish.
+
+## When to Use
+
+Describe scenarios when this skill applies.
+
+## How to Use
+
+Step-by-step instructions or patterns.
+
+## Examples
+
+Concrete examples demonstrating usage.
+
+## Reference
+
+- [Detailed Reference](references/REFERENCE.md) - Link to additional docs
+```
+
+## Progressive Disclosure
+
+Structure skills for efficient context usage:
+
+| Layer | Token Budget | When Loaded |
+|-------|--------------|-------------|
+| Metadata | ~100 tokens | At startup (all skills) |
+| Instructions | < 5000 tokens | When skill activated |
+| References | As needed | On demand |
+
+### Guidelines
+
+1. **Keep SKILL.md under 500 lines** - Move detailed content to references
+2. **Front-load key information** - Put most important patterns first
+3. **Use tables for quick reference** - Easy to scan
+4. **Link to references** - Don't inline everything
+
+## Optional Directories
+
+### scripts/
+
+Executable code that agents can run:
+
+```
+scripts/
+├── validate.sh       # Validation script
+├── generate.py       # Code generator
+└── setup.js          # Setup helper
+```
+
+Scripts should:
+- Be self-contained or document dependencies
+- Include helpful error messages
+- Handle edge cases gracefully
+
+### references/
+
+Additional documentation loaded on demand:
+
+```
+references/
+├── PATTERNS.md       # Common patterns
+├── API.md            # API reference
+└── EXAMPLES.md       # Extended examples
+```
+
+Keep individual reference files focused. Smaller files = less context usage.
+
+### assets/
+
+Static resources:
+
+```
+assets/
+├── template.xml      # File templates
+├── schema.json       # Schemas
+└── diagram.png       # Visual aids
+```
+
+## File References
+
+Use relative paths from the skill root:
+
+```markdown
+See [the reference guide](references/REFERENCE.md) for details.
+
+Run the setup script:
+scripts/setup.sh
+```
+
+Keep references one level deep. Avoid deeply nested chains.
+
+## Skill Categories
+
+### Developer Skills (`.claude/skills/`)
+
+Skills for contributors working on this codebase:
+
+- Command development patterns
+- Testing approaches
+- API client patterns
+- Documentation standards
+
+### User-Facing Skills (`plugins/*/skills/`)
+
+Skills for users of the tool:
+
+- CLI command usage
+- Platform-specific patterns (B2C Commerce)
+- Integration guides
+
+## Writing Effective Skills
+
+### 1. Start with the User's Goal
+
+```markdown
+## Overview
+
+This skill helps you [accomplish X] by [doing Y].
+```
+
+### 2. Provide Quick Reference Tables
+
+```markdown
+| Command | Description |
+|---------|-------------|
+| `cmd1`  | Does X      |
+| `cmd2`  | Does Y      |
+```
+
+### 3. Show Concrete Examples
+
+```markdown
+## Examples
+
+### Basic Usage
+
+\`\`\`bash
+b2c command --flag value
+\`\`\`
+
+### Advanced Usage
+
+\`\`\`bash
+b2c command --complex-flag
+\`\`\`
+```
+
+### 4. Explain When NOT to Use
+
+```markdown
+## When NOT to Use
+
+- Scenario A (use skill-x instead)
+- Scenario B (manual approach better)
+```
+
+### 5. Link to Authoritative Sources
+
+Reference official documentation rather than duplicating it:
+
+```markdown
+## Reference
+
+For complete API documentation, see [Official Docs](https://example.com/docs).
+```
+
+## Validation Checklist
+
+Before publishing a skill:
+
+- [ ] Frontmatter has `name` and `description`
+- [ ] SKILL.md under 500 lines
+- [ ] Key information appears early
+- [ ] Examples are concrete and runnable
+- [ ] Reference links are valid
+- [ ] No deeply nested reference chains
+- [ ] Tested with target agent
+
+## Detailed Reference
+
+- [Patterns and Examples](references/PATTERNS.md) - Patterns from B2C skills