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/34-seo-reporting-dashboard/desktop/tools/semrush.md

@@ -0,0 +1,43 @@
+# Semrush MCP
+
+In the reporting dashboard, Semrush is the **default for organic traffic, traffic value, visibility, and period-over-period traffic deltas**.
+
+Call pattern: discovery → `get_report_schema` → `execute_report`.
+
+## Tools used in dashboard sections
+
+| Section | Report |
+|---|---|
+| Health score header (organic traffic) | `overview_research` |
+| Traffic value (USD) | `overview_research` |
+| Visibility / ranking distribution | `tracking_research`, `organic_research` |
+| Period-over-period delta | `trends_research` |
+| Competitor benchmark cards | `organic_research` (per competitor) |
+| Authority Score (DR alternate) | `backlink_research` |
+
+## Example
+
+```
+mcp__semrush__overview_research(query="<domain>", database="us")
+mcp__semrush__trends_research(query="<domain>", database="us")
+mcp__semrush__organic_research(query="<competitor>", database="us")
+```
+
+Korean market: `database="kr"`. Always state the database used in the dashboard's Sources subsection.
+
+## Strengths for the dashboard
+
+- Cleanest combined traffic + traffic-value + visibility snapshot per domain.
+- `trends_research` returns period-over-period directly — feeds delta arrows in the executive summary.
+- Cross-database calls enable multi-market dashboards without per-market vendor switching.
+
+## Reporting rule
+
+Whenever Semrush feeds a section, list it in the report's **Sources** subsection (e.g., `Organic traffic: Semrush overview_research (database=us)`). Keep `database` value visible — it changes the numbers.
+
+## Not for this skill when
+
+- **Backlinks / DR section** — Ahrefs.
+- **AI visibility / Brand Radar section** — Ahrefs.
+- **First-party clicks / impressions / CTR** — GSC.
+- **GA4 / GBP / technical-health sections** — OurSEO CLI.