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/21-seo-position-tracking/desktop/tools/ahrefs.md

@@ -1,15 +1,27 @@
-# Ahrefs
+# Ahrefs MCP
 
-> TODO: Document tool usage for this skill
+**Default** when an Ahrefs Rank Tracker project exists for the domain — Ahrefs' rank tracker is the strongest in this category. Namespace: `mcp__ahrefs__*` / `mcp__claude_ai_Ahrefs__*`.
 
-## Available Commands
+## Key endpoints
 
-- [ ] List commands
+- `rank-tracker-overview` — current position distribution, visibility
+- `rank-tracker-serp-overview` — SERP for each tracked keyword
+- `rank-tracker-competitors-overview` — competitor positions for the same keywords
+- `rank-tracker-competitors-pages` — top competitor pages
+- `rank-tracker-competitors-stats` — aggregate competitor visibility
+- `gsc-keywords` — first-party rank data when GSC is connected to the Ahrefs project
 
-## Configuration
+## Example
 
-- [ ] Add configuration details
+```
+mcp__ahrefs__rank-tracker-overview(project_id="<id>")
+mcp__ahrefs__rank-tracker-competitors-stats(project_id="<id>")
+```
 
-## Examples
+Project must already exist in Ahrefs — list via `management-projects`.
 
-- [ ] Add usage examples
+## Not for this skill when
+
+- **No Ahrefs project for the domain** — Semrush `tracking_research` is the simplest alternative.
+- **First-party rank truth without an Ahrefs project** — use GSC via `our research search-console queries`.
+- **One-off spot-check for a single keyword** — `mcp__ourseo__check_serp` is cheaper.