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/32-seo-crawl-budget/desktop/tools/ahrefs.md

@@ -1,17 +1,33 @@
-# Ahrefs MCP Tools
+# Ahrefs MCP
 
-## site-explorer-pages-history
+In crawl-budget work, Ahrefs Site Audit surfaces **orphans, redirect chains, parameter URLs, and soft 404s** without running your own crawler. Useful when an Ahrefs Site Audit project already exists for the domain.
 
-Get historical page data for a domain to compare indexed pages with crawled pages.
+## Key endpoints
+
+- `site-audit-projects` — list available projects
+- `site-audit-issues` — current crawl-budget-relevant issues (redirect chains, broken links, parameter URLs)
+- `site-audit-page-explorer` — per-URL issue surfacing
+- `site-audit-page-content` — content-level signals (thin content, duplicates)
+- `site-explorer-pages-history` — page count over time vs. observed bot crawls
+
+## Example
 
 ```
-mcp__ahrefs__site-explorer-pages-history
+mcp__ahrefs__site-audit-projects()
+mcp__ahrefs__site-audit-issues(project_id="<id>")
+mcp__ahrefs__site-audit-page-explorer(project_id="<id>")
+mcp__ahrefs__site-explorer-pages-history(target="<domain>", history="weekly")
 ```
 
-**Parameters**:
-- `target` (string, required): Domain or URL to analyze
-- `date_from` (string): Start date (YYYY-MM-DD)
-- `date_to` (string): End date (YYYY-MM-DD)
-- `mode` (string): "domain", "prefix", "exact", "subdomains"
+## Strengths for this skill
 
-**Use case**: Compare Ahrefs indexed page counts with server log crawl data to identify indexing gaps and crawl budget inefficiencies.
+- Pre-computed orphan detection and redirect-chain surfacing.
+- Pages-history helps validate "are we crawling pages that don't exist anymore?"
+- Cross-reference Ahrefs indexed-page count with server log crawl data to spot indexing gaps.
+
+## Not for this skill when
+
+- **Actual bot behaviour at the origin** — only server access logs show this. Use `scripts/log_parser.py` + `scripts/crawl_budget_analyzer.py`.
+- **First-party Googlebot crawl stats** — GSC Coverage / Crawl Stats reports.
+- **Naver / Daum / Kakao bot analysis** — Ahrefs covers Googlebot well but underrepresents Korean bots; rely on logs.
+- **No Ahrefs project exists for the domain** — fall back to Semrush `siteaudit_research` or OurSEO crawl.