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/23-seo-content-strategy/desktop/tools/semrush.md

@@ -0,0 +1,34 @@
+# Semrush MCP
+
+**Default** for organic content discovery and keyword expansion in content strategy work.
+
+Call pattern: discovery → `get_report_schema` → `execute_report`.
+
+## Key reports
+
+- `organic_research` — organic keywords + top pages for a domain
+- `keyword_research` — expansion, volume, KD, intent
+- `overview_research` — domain-level traffic/visibility
+
+## Example
+
+```
+mcp__semrush__organic_research(query="<competitor.com>", database="us")
+mcp__semrush__keyword_research(query="<seed topic>", database="us")
+mcp__semrush__get_report_schema(report_id="<id>")
+mcp__semrush__execute_report(report_id="<id>", params={...})
+```
+
+Korean market: `database="kr"`.
+
+## Strengths for this skill
+
+- Best overall keyword + organic competitor view.
+- Cleanest discovery → schema → execute pattern for batch content brief generation.
+- Pairs with OurSEO crawl (on-page) and GSC (first-party performance).
+
+## Not for this skill when
+
+- **On-page inventory of target site** — OurSEO `crawl_website`.
+- **Cannibalization detection** — GSC first-party `combined` report.
+- **Korean Naver content** — `our research naver *` covers Naver-specific patterns Semrush doesn't.