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/26-seo-international/desktop/skill.yaml

@@ -1,8 +1,16 @@
 name: seo-international
 description: |
   International SEO audit and hreflang validation. Triggers: hreflang, international SEO, multi-language, multi-region, content parity, x-default, ccTLD.
+
+# Allowed tools list every backend the skill can pull international-SEO data from.
+# Per-task selection happens in SKILL.md > Data Source Selection — NOT here.
 allowed-tools:
-  - mcp__ahrefs__*
+  - mcp__ourseo__*                # default — crawl + hreflang validation + parity check
+  - mcp__ahrefs__*                # site-audit-* for hreflang issues, web-analytics-countries-*, site-explorer-metrics-by-country
+  - mcp__claude_ai_Ahrefs__*      # Ahrefs (Claude.ai namespace)
+  - mcp__semrush__*               # siteaudit_research, organic_research by database
+  - mcp__dfs-mcp__*               # DataForSEO MCP fallback for per-country SERP
+  - Bash                          # `our` CLI — crawl, audit, hreflang fix
   - mcp__notion__*
   - WebSearch
   - WebFetch