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/20-seo-serp-analysis/desktop/tools/semrush.md

@@ -0,0 +1,28 @@
+# Semrush MCP
+
+Default backend for Google SERP overview, organic competitor positions, SERP-feature presence.
+
+Call pattern: discovery → `get_report_schema` → `execute_report`.
+
+## Key reports (SERP toolkit)
+
+- Domain overview (organic positions, traffic, distribution)
+- Position changes (new/lost/improved/declined)
+- Organic competitors per keyword
+- SERP-feature presence per keyword
+
+## Example
+
+```
+mcp__semrush__overview_research(query="<keyword>", database="us")
+mcp__semrush__get_report_schema(report_id="<id from above>")
+mcp__semrush__execute_report(report_id="<id>", params={"phrase": "<keyword>", "database": "us"})
+```
+
+Korean market: `database="kr"`. Default `database="us"` per Semrush MCP instructions.
+
+## Not for this skill when
+
+- **Naver SERP** — Semrush is Google-only. Use `our research naver serp`.
+- **Raw SERP JSON for parsing all features** (PAA list, video carousel items, image pack) — DataForSEO via `our serp live` exposes richer JSON.
+- **Backlink graph for ranking pages** — Ahrefs `site-explorer-*` is stronger.