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/32-seo-crawl-budget/desktop/tools/semrush.md

@@ -0,0 +1,30 @@
+# Semrush MCP
+
+Alternative third-party site audit when no Ahrefs project exists for the domain.
+
+Call pattern: discovery → `get_report_schema` → `execute_report`.
+
+## Key report
+
+- `siteaudit_research` — site-wide audit catching redirect chains, parameter URLs, thin content, broken links
+
+## Example
+
+```
+mcp__semrush__siteaudit_research(query="<domain>", database="us")
+mcp__semrush__get_report_schema(report_id="<id>")
+mcp__semrush__execute_report(report_id="<id>", params={...})
+```
+
+## When Semrush is useful here
+
+- No Ahrefs Site Audit project — Semrush is the quickest alternative.
+- User is already in Semrush context.
+- Cross-vendor verification of orphan / redirect-chain counts.
+
+## Not for this skill when
+
+- **Actual bot behaviour at the origin** — server access logs (Nginx/Apache/CloudFront) only.
+- **First-party Googlebot crawl stats** — GSC.
+- **Korean bot analysis** (Yeti/Naver, Daumoa/Kakao) — Semrush underrepresents these. Use logs.
+- **An Ahrefs project already exists** — prefer Ahrefs `site-audit-*` for historical continuity.