fix(commands,persist): Improve session listing UX when persist is disabled

[mhiro2]

Summary

• Added explicit user feedback when :PeekstackListSessions is executed while persist is disabled.
• Switched session timestamp formatting from os.date to vim.fn.strftime to align with Neovim APIs.

Changes

• d77dde2 — fix(persist): notify when list sessions is disabled
  • Added disabled-state guard to persist.list_sessions.
  • Added silent handling to control notification behavior.
  • Ensured early return with {} when persist is disabled.
  • Added/updated tests to verify:
    • notification is shown for :PeekstackListSessions when disabled
    • disabled list_sessions returns empty data and does not invoke async callback
• a8f4da7 — fix(commands): use strftime for session updated_at
  • Replaced os.date with vim.fn.strftime for session updated_at display.
  • Added test coverage to assert vim.fn.strftime is actually used.