Positioning docs: migration guides, uninstall guarantee, non-goals, native-Bases comparison

[dsebastien]

What: Four documentation deliverables (no code):

1. Uninstall guarantee: an explicit FAQ/README section — everything is frontmatter; removing the plugin loses nothing.
2. Native-Bases comparison: Obsidian's native Kanban view is in active development (roadmap) with Calendar planned. Publish a clear "what KAP adds above the native view" page (state machines, automations, relationships + rollups, WBS/Gantt/Triage/Agenda, GTD depth) so the differentiation is legible before the native MVP ships.

See documentation/research/feature-ideas-2026-08-21.md (round 2, "The strategic fact").