chore: remove dead cookiecutter regeneration infrastructure

The repo was bootstrapped from `cookiecutter-pylibrary`. The
regeneration scaffolding has been sitting around since then — unused,
out of date, and in some cases pointing at infrastructure (AppVeyor)
that hasn't been part of CI for years. Knock it all down in one
sweep so there's nothing dead-by-design in the tree after this PR.

Removed
- `.cookiecutterrc` — cookiecutter regen metadata.
- `ci/bootstrap.py` — entry point for regenerating workflow files
  from the templates.
- `ci/templates/` (whole directory):
  - `.appveyor.yml` template (AppVeyor isn't wired up).
  - `.github/workflows/github-actions.yml` Jinja template (the live
    workflow has evolved past it via #32, #35, #37).

Coupled updates
- `tox.ini`: dropped the `[testenv:bootstrap]` env (calls deleted
  `bootstrap.py`) and the `bootstrap` reference in the basepython
  selector.
- `MANIFEST.in`: removed `include .cookiecutterrc`.
- `setup.cfg`: removed `ci/templates` from the flake8 `exclude` and
  isort `skip` lists (dead exclusion paths).
- `.pre-commit-config.yaml`: removed `ci/templates` from the
  top-level `exclude` regex.
- `tests.local.sh`: replaced the per-version `if/elif` chain (which
  still listed Python 3.7 and missed 3.12-3.14) with a single
  programmatic `tox -e py$VERSION` lookup against
  `tox --listenvs-all`. Self-maintaining when the matrix changes.
- `CHANGELOG.rst`: added a Housekeeping bullet for this PR.

Live and untouched
- `ci/requirements.txt` is still installed by every CI workflow
  (`pip install -r ci/requirements.txt`). Not regen-related.

Closes #46

1 files changed, 0 insertions, 1 deletions

diff --git a/MANIFEST.in b/MANIFEST.in
index a7518d1..bd328f6 100644
--- a/MANIFEST.in
+++ b/MANIFEST.in
@@ -4,7 +4,6 @@ graft ci
 graft tests
 
 include .bumpversion.cfg
-include .cookiecutterrc
 include .coveragerc
 include .editorconfig
 include .github/workflows/github-actions.yml