fix(server): stabilize non-repository Git diagnostics

[EricTsai83]

What Changed

• Use LC_ALL=C only for the two Git commands whose non-repository control flow parses an English diagnostic.
• Add an integration test that verifies general Git subprocesses continue to preserve a caller-provided locale.

Why

T3 Code identifies a non-repository result by matching Git's English diagnostic text. Git localizes that diagnostic through gettext, while the subprocess previously inherited the host locale. On systems with Git translations installed, a normal non-repository status could therefore be misclassified as a GitCommandError.

Using the C locale at these two call sites makes the parsed diagnostic deterministic without changing locale behavior for unrelated Git operations. LC_ALL is scoped to the commands instead of using only LC_MESSAGES so the fix also works when the host already defines LC_ALL, which takes precedence over individual locale categories.

Validation

• vp check
• vp run typecheck
• vp test apps/server/src/vcs/GitVcsDriverCore.test.ts

Checklist

• This PR is small and focused
• I explained what changed and why
• No UI changes

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[EricTsai83]

Superseded by #4077, which uses the replacement fix/git-non-repository-locale branch and centralizes stable diagnostic execution across all three affected command paths. Closing this draft so review stays on the complete replacement.