Fixture git commands see nothing of the developer's environment (#423)

[TheAmericanMaker]

What: tests/helpers/git-config-isolation.mjs becomes
tests/helpers/git-environment-isolation.mjs (git mv; all nine importers
and the test file follow). The side-effect import now applies
isolateGitEnvironment(), which keeps the four configuration sources from
#412/#421 and adds the rest of the class: GIT_TEMPLATE_DIR (deleted),
the repository-location overrides GIT_DIR, GIT_WORK_TREE, GIT_INDEX_FILE,
GIT_OBJECT_DIRECTORY, GIT_ALTERNATE_OBJECT_DIRECTORIES, GIT_COMMON_DIR,
GIT_NAMESPACE, GIT_CEILING_DIRECTORIES (deleted), the programs git would
launch GIT_PAGER, GIT_ASKPASS, GIT_SSH, GIT_SSH_COMMAND (deleted), and
GIT_EDITOR / GIT_SEQUENCE_EDITOR = ":" with GIT_TERMINAL_PROMPT=0.
isolateGitConfig and ABSENT_GIT_CONFIG keep their names. No core/ change.

Why: the helper was named for configuration and covered exactly that,
but GIT_TEMPLATE_DIR is not configuration and git copies it, hooks/
included, into every git init — so a developer's executable
hooks/pre-commit ran inside every fixture commit under "full" isolation.
The issue's Option 2: name the helper for what it guarantees so the next
member of the class is easier to see, and sweep the class now.

Class sweep (git 2.55, git help git ENVIRONMENT):
neutralize: GIT_TEMPLATE_DIR (proven: failing pre-commit breaks fixture
commit); GIT_DIR (proven: git -C fixture init + remote add land
in the exported repo, fixture never gets a .git); GIT_INDEX_FILE
(proven: git add writes the exported index, fixture status shows
??); GIT_EDITOR (proven: commit --amend runs the editor and fails
with it); GIT_WORK_TREE / GIT_OBJECT_DIRECTORY / GIT_ALTERNATE_* /
GIT_COMMON_DIR / GIT_NAMESPACE / GIT_CEILING_DIRECTORIES (same
mechanism as GIT_DIR: redirect the repository a command acts on);
GIT_SEQUENCE_EDITOR / GIT_PAGER / GIT_ASKPASS / GIT_SSH(COMMAND) /
GIT_TERMINAL_PROMPT (programs and prompts; fixtures never want one).
leave: GIT_AUTHOR* / GIT_COMMITTER_* (only change identity/date, no
test asserts on them, CI sets them deliberately); GIT_TRACE*,
GIT_FLUSH, GIT_ADVICE, GIT_PROGRESS_DELAY (stderr diagnostics, no
outcome change, wanted when debugging); GIT_DEFAULT_HASH /
GIT_DEFAULT_REF_FORMAT (on-disk format the developer chose for every
new repo; suite should pass under it); GIT_EXEC_PATH / PATH / HOME
(which git runs is out of scope); GIT_EXTERNAL_DIFF / GIT_DIFF_OPTS
(porcelain-only; fixtures use plumbing flags). No test in tests/
sets any neutralized variable deliberately (grep).

Tests (tests/git-environment-isolation.test.mjs, renamed with the
helper because it exercises the whole boundary, not configuration):
four new cases in the #421 shape — each first proves the vector is real
under configuration-only isolation, then runs a fresh child with the
variable injected and the helper imported first. The allow-list test
widens from ^GIT_CONFIG to ^GIT_ and requires the new members. Against
the main helper: 5/12 fail (6-9 and 12); the TEMPLATE_DIR control shows
template hook ran on the fixture commit's stderr.

Mutation: removing each neutralization line alone fails its own test
plus the allow-list (6+12, 7+12, 8+12, 9+12); removing the location
loop fails 7+8; applying isolateGitConfig() instead of
isolateGitEnvironment() at import fails 6-9. No survivors.

npm run build: 0 TS errors. npm test: 1276/1276 twice (was 1272).
git diff --check clean. Developer's global git config: 7 entries before
and after; the helper writes only process.env.

Closes #423