Fix logdone by writing CLOSED timestamps on TODO completion

[chadhs]

Summary

• Honor #+STARTUP: logdone / :LOGGING: logdone when completing TODOs by writing a CLOSED: planning timestamp (and remove it when leaving a done state)
• Fixes Closing TODOs does not log as per logdone #980 by matching Emacs org-log-done / time behavior for non-repeating tasks
• Document the new in-buffer options in README.org

Verification

Reproduced the reporter's exact fixture from #980:

#+STARTUP: logdone
* TODO clean up here in prep for going to alno

After advancing TODO → DONE, export contains:

* DONE clean up here in prep for going to alno
  CLOSED: [2024-04-17 Wed 15:03]

Unit tests cover STARTUP/property detection, CLOSED add/remove, SCHEDULED preservation, and the no-logdone default.

Test plan

• yarn test --testPathPattern='(org\.unit\.test|OrgFile\.unit\.test)'
• Reproduce Closing TODOs does not log as per logdone #980 fixture and confirm CLOSED: is written on completion
• Manually toggle TODO → DONE in the UI with #+STARTUP: logdone
• Confirm nologdone and missing logdone do not write CLOSED:
• Confirm un-completing a DONE header removes CLOSED: