v4.0.0-beta.21

What's Changed

• docs: refresh stale paths and references in AGENTS/README by @takkyun in #140
• docs: flip PR comment convention to English-first by @takkyun in #142
• test: harden config, charset, boundary defenses, and SB3 template compat by @takkyun in #141
• fix: harden FTS5 startup probe and OG card regeneration by @takkyun in #143
• docs(guide): add non-technical SB2 migration guide by @takkyun in #144
• fix(importer): keep SB2 parent link to category 0 by @takkyun in #145
• fix(importer): keep SB3 category parent when parent is category 0 by @takkyun in #146

Full Changelog: v4.0.0-beta.20...v4.0.0-beta.21