refactor(runway): compose the checkout git env from gitexec

[behinddwalls]

Summary

runGit in the Runway server's checkout provisioning kept its own copy of the scrubbed-plus-transport environment. Build it through gitexec.Env instead — the same source the merger uses — so provisioning and merging assemble an identical environment from one definition.

This also closes a gap the two hand-written copies had already opened: provisioning omitted GIT_ATTR_NOSYSTEM and the no-pager/no-editor settings the merge path sets, so a system gitattributes file could influence the initial checkout but not later merge operations. Both now run under the same scrub set.

Test Plan

✅ //service/runway/server green. Provisioning and merge paths now assemble an identical git environment from one source.

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

[mnoah1]

Should this include Passthrough: runtime.PassthroughEnv, matching the merger’s environment construction? Otherwise a custom transport variable works during merging but not during the provisioning fetch. The current server does not populate this field, but it leaves the two paths short of the stated identical environment.