The desktop's old recommendation inputs were the only client of
GET /api/mcp/catalog?detect_apps=true and the only sender of the guide_*
onboarding steps; both went with the old ::onboarding chain.
- The catalog route loses detect_apps; mcp_app_detection.py and its two
test files go. The manifest's suggest.applications validator moves into
mcp_catalog.py, which still validates applications / requires_app /
examples (documented in features/mcp.md).
- DESKTOP_ONBOARDING_STEPS and the v4 schema drop guide_connectors,
guide_first_build, guide_layout and guide_look. v3 stays as published.
- developer-guide/onboarding-recommendations.md described that pipeline
and is removed.
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