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/20-seo-serp-analysis/desktop/tools/ourseo.md

@@ -0,0 +1,39 @@
+# OurSEO Agent (CLI + MCP)
+
+Two paths for SERP work:
+
+1. **CLI** (Claude Code, Bash) — `our serp *` (DataForSEO Google) + `our research naver serp` (Naver-only).
+2. **MCP** (Claude Desktop) — `mcp__ourseo__check_serp` for single-keyword spot-check.
+
+## CLI commands
+
+```bash
+# Google SERP via DataForSEO
+our serp live "<keyword>" --location 2410 --language ko
+our serp competitors <domain> --location 2410
+our serp ranked-keywords <domain> --location 2410 --limit 50
+our serp domain-overview <domain> --location 2410
+
+# Naver SERP (Korean engines — Naver-only path)
+our research naver serp "<keyword>"
+our research naver serp "<keyword>" --domain <target.com>
+```
+
+Location codes: `2410` Korea, `2840` US, `2392` Japan.
+
+## MCP tool
+
+```
+mcp__ourseo__check_serp(keyword="<keyword>", domain="<target>", country="kr")
+```
+
+## Strengths
+
+- **Only** path that covers Naver SERP (blog, cafe, knowledge iN, Smart Store, brand zone, shortform, influencer).
+- Cheapest per call for batch Korean-market SERP work.
+- Full live SERP JSON with all features for export.
+
+## Not for this skill when
+
+- **Modelled SERP overview** (volume + CPC trend per keyword) — Semrush is cleanest for that.
+- **Brand Radar / AI search SERP** — Ahrefs Brand Radar is the only option.