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/31-seo-competitor-intel/desktop/tools/semrush.md

@@ -0,0 +1,35 @@
+# Semrush MCP
+
+**Default** for competitor organic profile, keyword overlap, traffic trends, and competitor auto-discovery.
+
+Call pattern: discovery → `get_report_schema` → `execute_report`.
+
+## Key reports
+
+- `organic_research` — organic keywords + competitor pulls for any domain
+- `trends_research` — period-over-period traffic deltas
+- `overview_research` — domain-level traffic/visibility for the comparison matrix
+- `backlink_research` — Authority Score and referring-domain counts (paired with Ahrefs for cross-check)
+
+## Example
+
+```
+mcp__semrush__organic_research(query="<target>", database="us")
+mcp__semrush__organic_research(query="<competitor>", database="us")
+mcp__semrush__trends_research(query="<target>", database="us")
+mcp__semrush__overview_research(query="<competitor>", database="us")
+```
+
+Korean market: `database="kr"`.
+
+## Strengths for this skill
+
+- **Auto-discovery** of organic competitors via `organic_research`.
+- Cleanest **keyword overlap** computation across target + competitors.
+- `trends_research` directly exposes period-over-period — required for threat scoring.
+
+## Not for this skill when
+
+- **Backlink-focused competitor comparison** — Ahrefs `site-explorer-organic-competitors` + `-backlinks-stats` is stronger.
+- **Korean Naver competitor presence** — Semrush is Google-only. Use `our research naver *` + WebSearch.
+- **SERP overlap spot-check** — `mcp__ourseo__check_serp` is cheaper.