review(wire): GitFileStatus ahead of the response type it serves

[CMGS]

A pure declaration move, split out of #130.

GitFileStatus is a method-less vocabulary type consumed by
GitStatusResult.Files. The house layout rule puts such a type ahead of the
first method-bearing type it serves, but it sat below GitStatusResult's whole
block instead. During #130 that region was being edited by #129, so the move was
held back to keep the diffs hunk-disjoint. #129 is merged, so it lands here.

Nothing but the declaration order changes: same fields, same JSON tags, same
godoc line. The shared fixture corpus still pins exactly 61 frames on both the
Rust and Go sides.

Gates

asl ./...                 0 findings
GOOS=linux asl ./...      0 findings
GOWORK=off make go-lint   ten "0 issues." lines (5 modules x 2 GOOS), fmt --diff silent
go test -race -count=1    18 packages ok across the 5 modules
go build ./...            sandboxd, sdk/go, e2e and mcp all still build

[CMGS]

Superseded by #131, which landed the same move on main.