refactor(git): relocate platform/gitexec under platform/git

[behinddwalls]

Summary

Move the shared git-exec package and its pinned-git test helper under a single platform/git umbrella: platform/gitexec becomes platform/git/exec, and platform/gitexec/gitexectest becomes platform/git/exectest — a sibling rather than nested, so the path no longer doubles "gitexec".

Package names are unchanged (gitexec, gitexectest), so this is an import-path move only: no call sites and no behavior change. It is the first step toward housing the change provider's bare-copy repo and auth plumbing alongside the shared git-exec package.

Test Plan

✅ bazel build/test of the moved packages and every importer; make check-gazelle clean; grep confirms no platform/gitexec (old path) imports remain.

Stack

1. @ refactor(git): relocate platform/gitexec under platform/git #609
2. feat(git/exec): a shared environment composer for git commands #610
3. refactor(changeprovider): make the git provider pure logic over a contract #611
4. refactor(merger): build the git command env from the shared composer #612
5. refactor(runway): compose the checkout git env from gitexec #613