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/ourseo.md

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+# OurSEO Agent (CLI + MCP)
+
+Two paths:
+
+1. **CLI** (Claude Code) — `our serp *` and `our keywords *` (DataForSEO under the hood).
+2. **MCP** (Claude Desktop) — `mcp__ourseo__check_serp` for single-keyword spot-check.
+
+## CLI commands
+
+```bash
+# Ranked keywords for a domain (modelled)
+our serp ranked-keywords <domain> --location 2410 --limit 100 --format json
+our serp domain-overview <domain> --location 2410 --format json
+our serp competitors <domain> --location 2410
+
+# Volume / KD pulls to weight visibility scores
+our keywords volume "<kw1>" "<kw2>" --location 2410 --language ko
+```
+
+Location codes: `2410` Korea, `2840` US, `2392` Japan.
+
+## MCP tool (spot-check)
+
+```
+mcp__ourseo__check_serp(keyword="<keyword>", domain="<target>", country="kr")
+```
+
+## Strengths
+
+- Cheapest per call for Korean batch rank pulls (`--location 2410`).
+- No project setup required — just call.
+- MCP `check_serp` is the cheapest live position verification path.
+
+## Not for this skill when
+
+- **Historical trend / period-over-period continuity** — needs an Ahrefs / Semrush project that polls regularly. OurSEO is point-in-time only; pair with MySQL workspace to keep history.
+- **First-party position truth** — GSC.