feat(git/exec): a shared environment composer for git commands

[behinddwalls]

Summary

Add Env(EnvOptions) as the single place that builds a git command's environment: the always-applied scrub set, plus transport variables inherited from the parent when set (SSH agent, PATH, TLS, proxy), plus caller-supplied literals appended last so they override. Re-express Command in terms of Env so there is one composer, not two.

This gives the change provider's repository and the Runway merger one source of truth to build on instead of each keeping its own copy of the scrub set and transport list. HOME is intentionally excluded from the shared transport list, since callers that isolate HOME and callers that inherit it disagree; each supplies it through Literal or Passthrough.

Test Plan

✅ new gitexec unit tests: scrub set always present, transport vars inherited only when set, literals override, and HOME is absent from the shared transport list. Existing Command consumers build unchanged.

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