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/31-seo-competitor-intel/desktop/tools/ourseo.md

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+# OurSEO Agent (CLI + MCP)
+
+For competitor intel, OurSEO contributes **SERP overlap spot-checks**, **content similarity / topic clustering**, and **Korean-market Naver competitor analysis** that Semrush/Ahrefs don't cover.
+
+## CLI commands (Claude Code)
+
+```bash
+# SERP overlap and competitor SERP pulls
+our serp competitors <target.com> --location 2410
+our serp ranked-keywords <competitor.com> --location 2410 --limit 100
+our serp domain-overview <competitor.com> --location 2410
+
+# Competitor crawl for content velocity / inventory
+our collect crawl https://<competitor> --max-pages 100
+
+# Korean engine competitor analysis (Naver)
+our research naver serp "<keyword>"
+our research naver keywords ideas "<keyword>" --limit 30
+our research keywords compare "<keyword>" --engines naver
+```
+
+## MCP tools (Claude Desktop)
+
+```
+mcp__ourseo__check_serp(keyword="<keyword>", domain="<target>", country="kr")
+mcp__ourseo__find_similar_pages(crawl_path="<path>", query="<topic>")
+mcp__ourseo__crawl_website(url="<competitor>", max_pages=50)
+```
+
+## Strengths for this skill
+
+- **Naver competitor analysis** — only path that covers Naver Blog/Cafe/Smart Store/Knowledge iN competitor presence.
+- **`find_similar_pages`** uses Vertex AI embeddings on prior crawl data — good for topic-cluster overlap detection between target and competitor.
+- **SERP spot-checks** are cheap; useful for ad-hoc "do we share top-10 with competitor X for keyword Y" questions.
+
+## Not for this skill when
+
+- **Backlink-graph competitor comparison** — Ahrefs.
+- **Modelled traffic / Authority Score** for the threat-scoring matrix — Semrush.
+- **Historical trend series** (DR over time, traffic over time) — Ahrefs/Semrush history endpoints.