docs: audit and refresh user-facing and internal docs

Cross-check every config key, shortcut, default, and command against the
current source and fix the drift (mpv.socketPath, auto_start_overlay
default, AniSkip TAB key, JLPT N4 color, secondary-sub font/defaults,
secondary-sub language behavior, modular mpv plugin layout, and more).
Add plain-language intros and first-use definitions across onboarding and
integration pages so non-technical readers can follow along.

Internal docs/: fix stale module paths in architecture/domains.md, add
missing contract entry points and catalog rows, and bump verified dates.
Remove the obsolete docs/plans/ directory (its only plan shipped in
0.15.0) and reframe planning.md so plans live with the work, not in docs/.

docs/knowledge-base/quality.md

@@ -37,4 +37,3 @@ Grades are directional, not ceremonial. The point is to keep gaps visible.
 
 - Some deep architecture detail still lives in `docs-site/architecture.md` and may merit later migration.
 - Quality grading is manual and should be refreshed when major refactors land.
-- Active plans can accumulate without lifecycle cleanup if humans do not prune them.