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/22-seo-link-building/desktop/skill.yaml

@@ -1,8 +1,14 @@
 name: seo-link-building
 description: |
   Link building diagnosis and backlink analysis. Triggers: backlink audit, link building, referring domains, toxic links, link gap, broken backlinks.
+
+# Allowed tools list every backend the skill can pull backlink data from.
+# Per-task selection happens in SKILL.md > Data Source Selection — NOT here.
 allowed-tools:
-  - mcp__ahrefs__*
+  - mcp__ahrefs__*                # default — backlink graph is Ahrefs' strongest moat
+  - mcp__claude_ai_Ahrefs__*      # Ahrefs (Claude.ai namespace)
+  - mcp__semrush__*               # backlink_research (alternative)
+  - mcp__ourseo__*                # gap: OurSEO has no backlink graph — Web/MCP only as supplement
   - mcp__notion__*
   - WebSearch
   - WebFetch