Remove created_at/updated_at from Task struct and frontmatter. Add a
version counter (u64) that increments on every write, defaults to 1 for
old files. list_tasks now groups by UUID and auto-deletes stale
duplicates (keeping highest version), preventing sync-induced dupes from
surfacing in the UI. has_time and parent are omitted from frontmatter
when false/null.
Update CLI, Tauri frontend types, and Svelte components to match.
Tristan Michael