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/23-seo-content-strategy/desktop/tools/ourseo.md

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+# OurSEO Agent (CLI + MCP)
+
+The **on-page** half of content strategy work — what the target site itself publishes, in what shape, with what schema. Pair with a keyword backend (Semrush/Ahrefs) for performance data.
+
+## CLI commands (Claude Code)
+
+```bash
+# Crawl the target for content inventory (titles, H1s, meta, last-modified)
+our collect crawl https://<site> --max-pages 500
+
+# Korean batch keyword research for content gaps
+our keywords ideas "<seed>" --location 2410 --limit 50
+our keywords for-site <competitor.com> --location 2410 --limit 100
+our serp ranked-keywords <domain> --location 2410 --limit 100
+
+# Naver-specific content discovery
+our research naver keywords ideas "<seed>" --limit 30
+our research naver serp "<keyword>"
+```
+
+## MCP tools (Claude Desktop)
+
+```
+mcp__ourseo__crawl_website(url="<target>", max_pages=200)
+mcp__ourseo__find_similar_pages(crawl_path="<path/to/crawl.json>", query="<topic>")
+mcp__ourseo__search_knowledge_graph(query="<entity>")
+```
+
+## Strengths for this skill
+
+- **`crawl_website` is the ground-truth on-page content inventory.** No vendor estimates — just what the site publishes.
+- **`find_similar_pages`** does semantic clustering on a prior crawl (Vertex AI embeddings) — useful for topic-cluster building and cannibalization candidates.
+- Korean Naver content discovery via `our research naver *` covers a market Semrush/Ahrefs don't reach.
+
+## Not for this skill when
+
+- **Search volume / KD per keyword** — Semrush MCP is the cleanest source.
+- **Top pages by traffic on competitor sites** — Ahrefs `site-explorer-top-pages`.
+- **First-party content performance** — GSC `pages` / `combined` reports.