docs(skills): add SKILL-MIGRATION-GUIDE + adopt root-SKILL.md hybrid as standard

- reference/SKILL-MIGRATION-GUIDE.md: one-page recipe to add a root SKILL.md to an
  existing skill (additive, no rewrite) — target structure, frontmatter checklist,
  a verified validation snippet, incremental-adoption guidance. References 16/17.
- CLAUDE.md: replace "Dual-Platform Skill Structure" with the hybrid (root SKILL.md +
  code/ + desktop/) as the going-forward standard; add "Root SKILL.md first" design
  principle; require a root SKILL.md for new skills; link the guide in Key Reference Files.
- SKILL-FORMAT-REQUIREMENTS.md: note the root-SKILL.md standard + cross-link the guide.

Validation snippet self-tested against 16/17 (frontmatter ≤1024, kebab name, all
referenced relative paths resolve).

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

reference/SKILL-FORMAT-REQUIREMENTS.md

@@ -1,5 +1,10 @@
 # Claude Skills - Format Requirements
 
+> **Going-forward standard:** each skill also gets a **root `SKILL.md`** (official Agent
+> Skills format, natively loadable) alongside the `code/` + `desktop/` packaging described
+> below. To add one to an existing skill, see `SKILL-MIGRATION-GUIDE.md`. This document
+> remains the spec for the dual-platform packaging layers.
+
 ## Overview
 
 This document describes the format requirements for Claude Skills in this repository. Skills support dual-platform development (Claude Code and Claude Desktop) with separated concerns.

reference/SKILL-MIGRATION-GUIDE.md

@@ -0,0 +1,87 @@
+# Skill Migration Guide — add a root `SKILL.md` (hybrid pattern)
+
+One page. How to make an existing dual-platform skill **natively loadable** as an Agent
+Skill without rewriting it. Reference implementations: `custom-skills/16-seo-schema-validator`
+and `custom-skills/17-seo-schema-generator`.
+
+## Why
+
+Claude Code / Desktop / API natively load the **Agent Skills format**: a directory whose
+**root** holds a `SKILL.md` (YAML frontmatter + body) plus optional bundled resources. Our
+older skills keep the directive in `desktop/SKILL.md` and `code/CLAUDE.md` — not at the
+root — so they aren't directly loadable. The fix is **additive**: promote a `SKILL.md` to
+the root. Keep `code/` and `desktop/` as-is. **No rewrite, no logic change.**
+
+## Target (hybrid) structure
+
+```
+XX-skill-name/
+├── SKILL.md            # ← NEW: root directive, official format (loadable)
+├── scripts/            # ← runnable scripts (single source of truth)
+├── references/         # ← heavy docs, loaded on demand
+├── templates/          # ← optional
+├── fixtures/           # ← optional (sample/test inputs)
+├── code/               # KEPT — code/CLAUDE.md redirects to ../SKILL.md + ../scripts
+└── desktop/            # KEPT — desktop/SKILL.md + skill.yaml + tools/
+```
+
+Scripts live **once** at the root `scripts/`; `code/CLAUDE.md` points to `../scripts/`
+(don't duplicate). Don't move existing scripts that other skills already reference.
+
+## Recipe (≈10 min/skill)
+
+1. **Copy** `desktop/SKILL.md` → root `SKILL.md`. (It's already frontmatter + body — the
+   right shape; it was just in the wrong place.)
+2. **Fix the frontmatter** against the checklist below.
+3. **Promote resources** to root siblings: if real scripts/docs live under `code/`, move
+   the canonical copy to root `scripts/` + `references/`; reference them from `SKILL.md`
+   with **relative paths** (`scripts/foo.py`, `references/bar.md`).
+4. **Redirect `code/CLAUDE.md`** to a short pointer: "canonical = `../SKILL.md`; scripts =
+   `../scripts/`" (see 16/17 `code/CLAUDE.md`).
+5. **Leave `desktop/`** intact (SKILL.md, skill.yaml, tools/). It stays the Desktop view.
+6. **Verify** (below). Commit.
+
+## Frontmatter checklist (the only hard requirement)
+
+```yaml
+---
+name: skill-name-kebab-case        # letters, numbers, hyphens ONLY
+description: |                     # third person; START with "Use when…" / what+when
+  What it does in one line. Triggers: kw1, kw2, 한국어 트리거.
+---
+```
+
+- [ ] `name`: kebab-case, no spaces/parens/special chars. (May keep the `NN-` prefix.)
+- [ ] `description`: third-person; says **when to use** + trigger keywords (EN + KO).
+- [ ] **Whole frontmatter ≤ 1024 characters.**
+- [ ] Body: concise markdown (aim < 500 words of always-on content); push detail into
+      `references/` and link it — don't `@`-include (that force-loads and burns context).
+- [ ] Optional fields allowed: `version`, `author`, `license`, `allowed-tools`, `metadata`.
+
+## Verify before committing
+
+```bash
+# frontmatter present + size sane
+head -20 custom-skills/XX-skill/SKILL.md
+python3 - <<'PY'
+import re,sys,pathlib
+t=pathlib.Path("custom-skills/XX-skill/SKILL.md").read_text()
+m=re.match(r'^---\n(.*?)\n---',t,re.S); assert m,"no frontmatter"
+fm=m.group(1); assert len(fm)<=1024,f"frontmatter {len(fm)} >1024"
+assert re.search(r'^name:\s*[a-z0-9-]+\s*$',fm,re.M),"name must be kebab-case"
+assert 'description:' in fm,"description required"
+print("OK: frontmatter valid")
+PY
+# every relative path referenced in SKILL.md must exist
+grep -oE '`(scripts|references|templates|fixtures)/[^`]+`' custom-skills/XX-skill/SKILL.md
+# if the skill has scripts, run its bundled sample/fixture end to end
+```
+
+## Adopt going forward
+
+- **New skills:** create a root `SKILL.md` from day one (the hybrid above). `code/` +
+  `desktop/` remain optional packaging, not the primary directive.
+- **Existing skills:** migrate **incrementally** — when you next touch a skill, apply the
+  recipe. A bulk migration of all skills isn't worth the churn unless you specifically
+  need them all loadable at once.
+- When in doubt, copy the shape of `16-seo-schema-validator` / `17-seo-schema-generator`.