chore(lint): enforce gofmt in CI and reformat the tree

golangci-lint had no formatter enabled, so gofmt drift accumulated
silently: 29 files were unformatted on main. The cost lands on review —
contributors' editors reformat whatever they touch, so alignment churn
rides along in feature diffs and has to be separated from real changes
every time. The same `Edge *edge.Client` realignment shows up in four
open PRs for exactly this reason, and two open PRs introduce new gofmt
failures that CI currently cannot see.

Enable the gofmt formatter and reformat the tree in one pass so the
next diff that touches these files is only the change itself.

No semantic changes: build, vet, and `go test ./... -race` (54 packages)
all pass.