fix(devkit): isolate git subprocesses from inherited GIT_* env

[crs48]

What

@xnetjs/devkit's NodeCommandRunner now scrubs git's repo-location environment variables (GIT_DIR, GIT_WORK_TREE, GIT_INDEX_FILE, and friends) for git invocations, so the explicit cwd is always authoritative. An explicit options.env entry still wins.

Why

RunOptions.cwd is required precisely so a caller "can never accidentally run in process.cwd()" — but git reads GIT_DIR/GIT_WORK_TREE/GIT_INDEX_FILE from the environment over cwd. A husky hook (pre-commit/pre-push) exports exactly those vars pointing at the hook's repo. So when the pre-push hook runs pnpm test, devkit's real-repo tests (git.test.ts) spawned git children that inherited the hook's GIT_DIR and operated on the actual worktree instead of their temp repo — running git config, git commit, and even git push against it.

This was not hypothetical: it clobbered a live worktree and its remote branch during PR #445 (and recurred on #444) — the worktree's user.name/email got overwritten and junk initial/checkpoint commits were pushed to origin.

How

• command-runner.ts: for command === 'git', merge { ...process.env, ...<git-location vars set to undefined>, ...options.env }. Node's spawn omits undefined-valued env keys, so the inherited leak is dropped while any explicit override is preserved. The scrubbed set is documented in the new GIT_LOCATION_ENV (module-internal — not added to the package barrel, so no public API change → patch).

Tests

• command-runner.test.ts: with a leaked bogus GIT_DIR, git rev-parse --is-inside-work-tree still resolves the cwd repo (would be fatal without the scrub); and an explicit options.env GIT_DIR still wins.
• git.test.ts: a checkpoint() lands in cwd while GIT_* points at a decoy, and the decoy never becomes a repo.
• Ran the two devkit test files directly (13 passing). Did not run the full suite locally, to avoid re-triggering the very leak this fixes on an unpatched checkout.

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[github-actions]

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