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/19-seo-keyword-strategy/desktop/tools/ahrefs.md

@@ -1,15 +1,54 @@
-# Ahrefs
+# Ahrefs MCP
 
-> TODO: Document tool usage for this skill
+Use Ahrefs when the user explicitly asks for Ahrefs data, when DR/UR weighting matters, or when the task needs `gsc-keywords` (Ahrefs is the only SEO MCP that exposes a Google Search Console integration directly).
 
-## Available Commands
+Namespace: `mcp__ahrefs__*` (Claude Desktop) / `mcp__claude_ai_Ahrefs__*` (Claude.ai). Same backend.
 
-- [ ] List commands
+## Keyword endpoints
 
-## Configuration
+- `keywords-explorer-overview` — volume, CPC, KD, parent topic
+- `keywords-explorer-matching-terms` — phrase-match expansion
+- `keywords-explorer-related-terms` — related ideas
+- `keywords-explorer-search-suggestions` — autocomplete-style suggestions
+- `keywords-explorer-volume-by-country` — country breakdown for one keyword
+- `keywords-explorer-volume-history` — historical search volume
 
-- [ ] Add configuration details
+## GSC integration (Ahrefs-only)
+
+- `gsc-keywords` — keywords the user's verified site actually ranks for (impressions, clicks, CTR, position)
+- `gsc-keyword-history` — historical performance of a query
+
+GSC endpoints require the Ahrefs project to be connected to the GSC property. Confirm with the user that the project exists in their Ahrefs workspace before relying on these.
+
+## Common parameters
+
+- `country` — `us`, `kr`, `jp`, etc. (lowercase ISO-2)
+- `keyword` — the seed term
+- `limit` — usually 30–100
 
 ## Examples
 
-- [ ] Add usage examples
+**Quick overview:**
+```
+mcp__ahrefs__keywords-explorer-overview(keyword="enterprise CRM", country="us")
+```
+
+**Related terms in Korean:**
+```
+mcp__ahrefs__keywords-explorer-related-terms(keyword="신라호텔", country="kr", limit=50)
+```
+
+**GSC first-party queries:**
+```
+mcp__ahrefs__gsc-keywords(project_id="<ahrefs project id>", limit=100)
+```
+
+## When NOT to use Ahrefs for this skill
+
+- Default keyword volume / matching terms — Semrush is the project default; only switch on explicit request.
+- Bulk Korean expansion — `our keywords ideas --location 2410` is usually cheaper.
+- Entity / Knowledge Graph seeding — `mcp__ourseo__search_knowledge_graph`.
+
+## Reference
+
+Always check `mcp__ahrefs__doc` once per session before first call — it documents current parameter shapes and may have changed.