refactor(platform): Move GitRuntime into gitworkspace/local as Runtime

[manjari25]

Why?

The pinned git runtime (executable, helper paths, scrubbed environment) is how the local workspace backend runs git, so it belongs in that backend rather than in the merger. The merger and the runway server wiring import it from there.

What?

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

Test Plan

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

Issues