refactor(platform): Move GitRuntime into platform/git/exec as Runtime

[manjari25]

Why?

The local workspace backend needs the same pinned runtime and scrubbed environment as the merger. This puts it in one shared place first.

What?

• Move the pinned git runtime (GitRuntime) from the merger into platform/git/exec as Runtime, with Validate() and Command().
• Switch the merger and the runway server wiring to gitexec.Runtime.
• Move the runtime tests next to the code.
• Validate now checks fields in a fixed order, so its error is deterministic. This is the only logic change.

Test Plan

• bazel test //platform/git/... //runway/extension/merger/... //service/runway/...
• make check-gazelle, make check-tidy and make lint pass

Issue

[behinddwalls]

i am wondering if it needs to exit on it's own vs being part of the local gitworkspace impl itself ? do you see any use of it outside?