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/gsc.md

@@ -0,0 +1,40 @@
+# Google Search Console
+
+**First-party position data** — what Google actually rendered for the verified site. Estimates from Semrush/Ahrefs/DataForSEO are modelled; GSC is observed.
+
+Two entry points:
+
+1. **`our research search-console` CLI** — primary, cached, scriptable.
+2. **Ahrefs `gsc-*`** — only when an Ahrefs project is connected to the GSC property.
+
+## CLI commands
+
+```bash
+our research search-console queries --site sc-domain:<domain> --days 28
+our research search-console pages --site sc-domain:<domain> --days 28
+our research search-console combined --site sc-domain:<domain> --days 28
+```
+
+`sc-domain:` prefix for Domain-verified properties. URL-prefix properties use the plain URL.
+
+## Ahrefs GSC tools
+
+```
+mcp__ahrefs__gsc-keywords(project_id="<id>", limit=100)
+mcp__ahrefs__gsc-keyword-history(project_id="<id>", keyword="<kw>")
+mcp__ahrefs__gsc-positions-history(project_id="<id>")
+mcp__ahrefs__gsc-performance-by-position(project_id="<id>")
+```
+
+## Strengths
+
+- **Ground truth** — only first-party position source.
+- Includes impressions and CTR per query — useful for visibility weighting beyond raw position.
+- Anonymous-query data via `mcp__ahrefs__gsc-anonymous-queries`.
+
+## Not for this skill when
+
+- **Net-new keyword discovery** — GSC only shows queries the site already ranked for. Pair with Semrush/Ahrefs for discovery.
+- **Competitor positions** — GSC is single-site.
+
+Gotcha: `our-claude-skills/custom-skills/15-seo-search-console/code/gotcha/gsc-cli-integration.md`.