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/25-seo-kpi-framework/desktop/tools/semrush.md

@@ -0,0 +1,40 @@
+# Semrush MCP
+
+In the KPI framework, Semrush is the **default for organic traffic + visibility + competitor KPIs**.
+
+Call pattern: discovery → `get_report_schema` → `execute_report`.
+
+## KPI dimensions served by Semrush
+
+| KPI | Report |
+|---|---|
+| Organic traffic (estimate) | `overview_research` |
+| Traffic value (USD) | `overview_research` |
+| Visibility / ranking distribution | `tracking_research`, `organic_research` |
+| Top organic keywords | `organic_research` |
+| Authority Score (DR alternative) | `backlink_research` |
+| Competitor organic share | `organic_research` |
+| Period-over-period traffic delta | `trends_research` |
+
+## Example
+
+```
+mcp__semrush__overview_research(query="<domain>", database="us")
+mcp__semrush__tracking_research(query="<keyword>", database="us")
+mcp__semrush__trends_research(query="<domain>", database="us")
+```
+
+Korean market: `database="kr"`.
+
+## Strengths for this skill
+
+- Cleanest combined traffic-value + visibility dashboard per domain.
+- `trends_research` exposes period-over-period directly.
+- Default backend for the project's SEO KPI map.
+
+## Not for this skill when
+
+- **Backlink / DR KPI** — Ahrefs is the default for this dimension.
+- **First-party clicks/impressions/CTR** — GSC via `our research search-console`.
+- **GA4 KPI** (sessions, conversions, bounce) — `our research ga4 *`.
+- **Local KPI (GBP)** — `our collect gbp *` / `our audit local`.