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/skill.yaml

@@ -1,14 +1,16 @@
-# Skill metadata (extracted from SKILL.md frontmatter)
-
 name: seo-serp-analysis
 description: |
   SERP analysis for Google and Naver. Triggers: SERP analysis, search results, featured snippet, SERP features, Naver SERP.
 
-# Optional fields
+# Allowed tools list every backend the skill can pull SERP data from.
+# Per-task selection happens in SKILL.md > Data Source Selection — NOT here.
 allowed-tools:
-  - mcp__ahrefs__*
+  - mcp__semrush__*               # default for Google SERP overview
+  - mcp__ahrefs__*                # Ahrefs serp-overview
+  - mcp__claude_ai_Ahrefs__*      # Ahrefs (Claude.ai namespace)
+  - mcp__ourseo__*                # OurSEO check_serp, audit_page
+  - mcp__dfs-mcp__*               # DataForSEO MCP fallback
+  - Bash                          # `our serp *` + `our research naver serp` (Claude Code only)
   - mcp__notion__*
   - WebSearch
   - WebFetch
-
-# triggers: []  # TODO: Extract from description