feat: add skill update detection via SHA-256 content hashing

- Add content_hash and updated_at fields to skills table (DB migration v6→v7)
- Compute directory content hash on install/import/restore for version tracking
- Add check_updates command: downloads repos, compares hashes, returns update list
- Add update_skill command: backs up old files, re-downloads and replaces SSOT
- Backfill content_hash for existing skills on first update check
- Add "Check Updates" button and per-skill update badge/button in UnifiedSkillsPanel
- Add i18n keys for zh/en/ja

src-tauri/src/lib.rs

@@ -963,6 +963,8 @@ pub fn run() {
             commands::scan_unmanaged_skills,
             commands::import_skills_from_apps,
             commands::discover_available_skills,
+            commands::check_skill_updates,
+            commands::update_skill,
             // Skill management (legacy API compatibility)
             commands::get_skills,
             commands::get_skills_for_app,