feat(seo-skills): multi-backend Data Source Selection (#7)

Replaces single-vendor (Ahrefs-only) tool defaults with a per-task
backend menu across all 14 SEO skills. Each skill now lists every
capable MCP in allowed-tools and documents how to pick between
Semrush, Ahrefs, OurSEO Agent (CLI + MCP), DataForSEO, and GSC
in its SKILL.md Data Source Selection section.

Tool stubs (~40 new files) populated per skill with capability
deltas, call patterns, and explicit "not for this skill when"
callouts so the menu is self-correcting.

Skills affected: 19-keyword-strategy, 20-serp-analysis,
21-position-tracking, 22-link-building, 23-content-strategy,
24-ecommerce, 25-kpi-framework, 26-international, 27-ai-visibility,
28-knowledge-graph, 31-competitor-intel, 32-crawl-budget,
33-migration-planner, 34-reporting-dashboard.

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

custom-skills/26-seo-international/desktop/tools/ourseo.md

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+# OurSEO Agent (CLI + MCP)
+
+**Default** for hreflang validation, content parity, redirect logic, and the fix engine. Only backend that owns the hreflang **fix** path end-to-end.
+
+## CLI commands (Claude Code)
+
+```bash
+# Crawl all language versions
+our collect crawl https://<site> --max-pages 500
+
+# Technical audit including hreflang
+our audit tech https://<site>
+
+# Hreflang fix (writes back to CMS if --deploy and CMS adapter configured)
+our fix hreflang --site https://<site>
+
+# Per-region SERP / Naver expansion checks
+our research naver serp "<keyword>"
+our research keywords compare "<keyword>" --engines naver
+```
+
+## MCP tools (Claude Desktop)
+
+```
+mcp__ourseo__crawl_website(url="<site>", max_pages=500)
+mcp__ourseo__audit_page(url="<url>", audit_type="tech")
+```
+
+## Strengths for this skill
+
+- **Hreflang validation:** bidirectional links, self-reference, x-default, language/region code validation.
+- **Hreflang fix:** auto-corrects tags via Ghost/WordPress/Strapi adapters.
+- **Content parity audit:** page counts, key-page availability, freshness comparison across language versions.
+- **Naver / regional engine coverage** — Semrush/Ahrefs only cover Google.
+
+## Not for this skill when
+
+- **Per-country traffic distribution KPI** — Ahrefs `web-analytics-countries` is cleaner.
+- **Per-database organic visibility comparison** — Semrush per-`database` calls.
+- **Redirect-logic audit with custom Accept-Language headers** — WebFetch directly.
+
+See also: `docs/user-manual.md` for the full `our fix` command surface.

custom-skills/26-seo-international/desktop/tools/semrush.md

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+# Semrush MCP
+
+Use for **per-database organic visibility** across markets — Semrush's database-per-market model is well-suited to international SEO comparison.
+
+Call pattern: discovery → `get_report_schema` → `execute_report`.
+
+## Per-market comparison pattern
+
+```
+# Pull organic positions per database (one call per target market)
+mcp__semrush__organic_research(query="<domain>", database="us")
+mcp__semrush__organic_research(query="<domain>", database="kr")
+mcp__semrush__organic_research(query="<domain>", database="jp")
+mcp__semrush__organic_research(query="<domain>", database="cn")
+mcp__semrush__organic_research(query="<domain>", database="de")
+
+# Cross-market visibility deltas surface content-parity gaps from a ranking perspective
+```
+
+## Key reports
+
+- `organic_research` — keywords/positions per `database`
+- `overview_research` — domain traffic per market
+- `siteaudit_research` — site-wide audit (catches hreflang issues with limitations)
+- `keyword_research` — per-market keyword expansion
+
+## Strengths for this skill
+
+- Quickest "where am I ranking in market X vs market Y" comparison.
+- Easy cross-database competitor pulls.
+
+## Not for this skill when
+
+- **Hreflang validation / parity audit / fix** — OurSEO is the default; only it owns the fix engine.
+- **Forced-redirect / Accept-Language audit** — WebFetch with explicit headers.
+- **Naver / regional engines** — Semrush is Google-only.