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/gsc.md

@@ -0,0 +1,45 @@
+# Google Search Console
+
+GSC's Coverage + Crawl Stats reports are the **first-party Googlebot view** — what Google considers crawled, indexed, or excluded. Pair with server access logs (the origin view) and OurSEO crawl (the published view) for a complete crawl-budget audit.
+
+## CLI access
+
+```bash
+our research search-console queries --site sc-domain:<domain> --days 28
+our research search-console pages --site sc-domain:<domain> --days 28
+```
+
+`sc-domain:` prefix for Domain-verified properties.
+
+Note: full **Coverage** and **Crawl Stats** report data is mostly UI-only in GSC — the API exposes performance (queries / pages) but limited crawl-stats fields. For exhaustive crawl analysis, export via the GSC UI and parse alongside log output.
+
+## Ahrefs GSC alternates
+
+When the site is connected as an Ahrefs project:
+
+```
+mcp__ahrefs__gsc-pages(project_id="<id>")
+mcp__ahrefs__gsc-pages-history(project_id="<id>")
+mcp__ahrefs__gsc-positions-history(project_id="<id>")
+```
+
+## How GSC pairs with the rest
+
+| Source | Shows |
+|---|---|
+| Server logs | What hit the origin (bot behaviour at HTTP level) |
+| OurSEO crawl | What URLs exist on the site today |
+| Sitemap | What URLs the site claims to publish |
+| **GSC** | **What Google considers indexed / excluded** |
+
+The intersection (and differences) reveal:
+- **Crawled-not-indexed**: Google saw it, won't index — often thin content or duplicates.
+- **Indexed-not-crawled-recently**: bot budget moved elsewhere.
+- **Discovered-not-crawled**: Google knows about the URL but hasn't fetched it.
+
+## Not for this skill when
+
+- **Korean bots** (Yeti, Daumoa) — GSC is Google-only.
+- **Bot User-Agent + IP analysis** — server logs.
+
+Gotcha: `our-claude-skills/custom-skills/15-seo-search-console/code/gotcha/gsc-cli-integration.md`.