chore: Minor hardening, performance, and type annotation improvements

[mhiro2]

Summary

• Add nil-safe accessors for session meta/items in the list sessions command
• Early-return in CursorMoved autocmd for non-peekstack windows to skip unnecessary work
• Cache scope_path base directory creation to avoid repeated ensure_dir calls
• Add EmmyLua type annotations for peek API, render win_opts, and popup model