chore: drop CHANGELOG.md again, and file #2's entry as a fragment

[CSSFrancis]

#2 was rebased onto main by restoring CHANGELOG.md, which #6 had replaced
with CHANGELOG.rst. The code in #2 is untouched and correct — this is only
about the conflict resolution.

Three things followed from it:

• main carried two changelogs, and the .md one was the copy from testing: remove the temp profile dir on close; pythonEnv tests clean up #2's
  merge base (1b369c0), which predates the 0.2.2 release — so it had lost the
  entire 0.2.2 section: the rectangle widget, the text labels, the readout,
  the anyplotlib 0.10.0 floor. CHANGELOG.rst still has all of it, so nothing
  is gone for good, but the two files disagreed about what shipped.
• testing: remove the temp profile dir on close; pythonEnv tests clean up #2's own entry went into the dead file instead of a fragment, so the next
  release notes would not have mentioned it at all. Before this PR,
  towncrier build --draft listed only ci: a Prepare Release workflow, so the tag and __version__ agree by construction #7 and the towncrier setup — Michael's
  fix was invisible.
• The link refs at the foot of the .md still pointed at v0.2.1...HEAD.

The fix

The resolution the rebase wanted: CHANGELOG.md stays deleted, and #2's
paragraph becomes upcoming_changes/2.bugfix.rst.

After this, towncrier build --draft --version 0.2.3 gives:

Bug Fixes
---------

- An end-to-end launch through the testing harness left its temporary Electron
  profile directory behind — a few megabytes per launch, and a downstream app's
  full suite added about 150. ``closeApp`` now removes it once the process is
  down, on the clean-close, hard-kill and no-app paths alike, and ``launchApp``
  reports the path as ``profileDir``. (`#2 <...pull/2>`_)

Maintenance
-----------

- A **Prepare Release** workflow … (`#7 <...pull/7>`_)
- The changelog is now assembled by towncrier …

Note for #4

#4's branch was rebased the same way — its head carries CHANGELOG.md | 11 + instead
of a fragment, so merging it as-is puts the same file back. There is a rebased
version ready that carries upcoming_changes/4.bugfix.rst instead.