feat(skills): bulk-add root SKILL.md to 61 skills (native Agent Skills loadability)

Run the additive migration pass from SKILL-MIGRATION-GUIDE: generate a root SKILL.md
for every skill that lacked one, copied from its desktop/SKILL.md (or code/SKILL.md),
with name set to the directory name and description + body preserved verbatim.

- scripts/migrate_skill_root.py: the reusable, non-destructive migrator (dry-run default).
- 61 new root SKILL.md (desktop source for most; code/SKILL.md for 61/62/92).
- Untouched: 16/17/95 (already had root); desktop/ and code/ packaging left intact.
- All 64 root SKILL.md validate: frontmatter <=1024, kebab name, description present.

Still MANUAL (no SKILL.md source — commands/README only), need hand-authored root SKILL.md:
81-mac-optimizer, 90-reference-curator, 91-multi-agent-guide, 94-dintel-bootstrap.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

custom-skills/13-seo-on-page-audit/SKILL.md

@@ -0,0 +1,103 @@
+---
+name: 13-seo-on-page-audit
+description: |
+  On-page SEO analyzer for meta tags, headings, links, images, and Open Graph.
+  Triggers: on-page SEO, meta tags, title tag, heading structure, alt text.
+---
+
+# SEO On-Page Audit
+
+## Purpose
+
+Analyze single-page SEO elements: meta tags, heading hierarchy, internal/external links, images, and social sharing tags.
+
+## Core Capabilities
+
+1. **Meta Tags** - Title, description, canonical, robots
+2. **Headings** - H1-H6 structure and hierarchy
+3. **Links** - Internal, external, broken detection
+4. **Images** - Alt text, sizing, lazy loading
+5. **Social** - Open Graph, Twitter Cards
+
+## MCP Tool Usage
+
+```
+mcp__firecrawl__scrape: Extract page HTML and metadata
+mcp__perplexity__search: Research SEO best practices
+mcp__notion__create-page: Save audit findings
+```
+
+## Workflow
+
+1. Scrape target URL with Firecrawl
+2. Extract and analyze meta tags
+3. Map heading hierarchy
+4. Count and categorize links
+5. Check image optimization
+6. Validate Open Graph tags
+7. Generate recommendations
+
+## Checklist
+
+### Meta Tags
+- [ ] Title present (50-60 characters)
+- [ ] Meta description present (150-160 characters)
+- [ ] Canonical URL set
+- [ ] Robots meta allows indexing
+
+### Headings
+- [ ] Single H1 tag
+- [ ] Logical hierarchy (no skips)
+- [ ] Keywords in H1
+
+### Links
+- [ ] No broken internal links
+- [ ] External links use rel attributes
+- [ ] Reasonable internal link count
+
+### Images
+- [ ] All images have alt text
+- [ ] Images are appropriately sized
+- [ ] Lazy loading implemented
+
+### Open Graph
+- [ ] og:title present
+- [ ] og:description present
+- [ ] og:image present (1200x630)
+
+## Output Format
+
+```markdown
+## On-Page Audit: [URL]
+
+### Meta Tags: X/5
+| Element | Status | Value |
+|---------|--------|-------|
+
+### Headings: X/5
+- H1: [text]
+- Hierarchy: Valid/Invalid
+
+### Links
+- Internal: X
+- External: X
+- Broken: X
+
+### Recommendations
+1. [Priority fixes]
+```
+
+## Limitations
+
+- Single page analysis only
+- Cannot detect JavaScript-rendered content issues
+- External link status requires additional crawl
+
+## Notion Output (Required)
+
+All audit reports MUST be saved to OurDigital SEO Audit Log:
+- **Database ID**: `2c8581e5-8a1e-8035-880b-e38cefc2f3ef`
+- **Properties**: Issue (title), Site (url), Category, Priority, Found Date, Audit ID
+- **Language**: Korean with English technical terms
+- **Audit ID Format**: [TYPE]-YYYYMMDD-NNN
+