fix(persist): Bind session writes to their repository and stack

[mhiro2]

Summary

• Session operations now write to the repository store that was current when they were requested, so a :cd during an async save, delete or rename no longer overwrites another repository's sessions.
• The session cache is kept per store file, so listing sessions after a directory change shows the current repository's sessions.
• Auto save remembers which stack (and store) it is mirroring, so quitting from an unrelated empty split no longer replaces the auto session with an empty stack.

Changes

• cd42383 : test(user_events): run session event tests in a throwaway repository
  • The save/restore event tests no longer share a store file with persist_sessions_spec running in a parallel process, which could overwrite its fixtures.
• 894ebeb : fix(persist): bind each session operation to the store it was requested in
  • Save, restore, list, delete and rename resolve the store file once and carry it through the update queue, read, write and cache refresh.
  • The session cache is keyed by store file.
  • Real-file tests in two temporary repositories cover save, delete, rename and queued updates across a directory change.
• eace374 : fix(persist): save the tracked stack on leave instead of the current window
  • The leave-time save writes the most recently changed stack to its own store, and does nothing if no stack changed during the session.
  • A pending debounced save is written out before a change for another stack or store replaces it, and each timer callback only saves the target it debounced.
  • Once the tracked root window has been closed, the save is skipped instead of saving the current window's stack. README and help describe the auto session target.