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/33-seo-migration-planner/desktop/tools/ourseo.md

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+# OurSEO Agent (CLI + MCP)
+
+**Default** for URL inventory, on-page crawl, technical audit, and index-status baseline. Distributed crawler scales to large catalogs.
+
+## CLI commands (Claude Code)
+
+```bash
+# URL inventory crawl (pre-migration)
+our collect crawl https://<old-site> --max-pages 5000
+our collect distributed https://<old-site> --workers 8 --max-pages 50000
+
+# Technical audit (catches redirect chains, canonical issues that affect migration)
+our audit tech https://<old-site>
+
+# Index baseline
+our research google index --domain <old-site>
+
+# Schema baseline (preserve schema across migration)
+our analyze mysql-batch --session <id> --export missing-schema --format excel
+
+# Post-migration: re-run on new domain and diff
+our collect crawl https://<new-site> --max-pages 5000
+our research google index --domain <new-site>
+our audit tech https://<new-site>
+```
+
+## MCP tools (Claude Desktop)
+
+```
+mcp__ourseo__crawl_website(url="<site>", max_pages=5000)
+mcp__ourseo__check_index(domain="<site>")
+mcp__ourseo__audit_page(url="<url>", audit_type="tech")
+```
+
+## Strengths for this skill
+
+- **Distributed crawl** with URL-hash partitioning — handles catalogs >50K URLs.
+- **Resume support** (`--resume`) — critical for multi-day baseline captures.
+- **MySQL workspace** stores baseline crawl; post-migration diff is a SQL query against the same DB.
+- **Index status spot-check** via `check_index` — finds URLs Google still has indexed from the old domain.
+
+## Pair with Firecrawl
+
+Firecrawl (`mcp__firecrawl__*`) is already in this skill's `allowed-tools`:
+
+- Use **OurSEO** as the default crawler for most sites.
+- Switch to **Firecrawl** when the target is JS-heavy (React/Vue/Angular SPA) and OurSEO's HTTP-level crawler under-reports URLs.
+- Use Firecrawl `firecrawl_scrape` for per-URL **redirect verification** post-launch (cheaper than re-crawling the whole site).
+
+## Not for this skill when
+
+- **Per-URL traffic value for risk scoring** — Ahrefs `site-explorer-top-pages` / Semrush `url_research`.
+- **First-party impression baseline** — GSC `pages` (REQUIRED for accurate post-migration diff).
+- **GA4 traffic baseline** — `our research ga4 *` (covered in OurSEO CLI but distinct from crawl path).