chore: bump Go to 1.27 and build tools

[umputun]

updates the build image to Go 1.27 and refreshes its pinned tools.

build.go/Dockerfile

• golang:1.26-alpine to golang:1.27-alpine
• GOLANGCI 2.12.2 to 2.13.1, the first release with Go 1.27 support
• GORELEASER 2.13.3 to 2.18.0
• STATIK 0.1.7 to 0.1.8

GOVERALLS stays at 0.0.12. The two Go 1.26 references in CLAUDE.md now match the image.

the built image reports Alpine 3.24.1 and Go 1.27.0. The installed golangci-lint, GoReleaser, statik and goveralls binaries match their declared versions.

moq remains intentionally installed from @latest; this build resolved it to v0.7.1. The golang:1.27-alpine tag and apk add inputs also float, so later rebuilds may produce different bytes without a source change.

the second commit records feature-branch push runs that report success without building an image, in docs/backlog/workflow-vacuous-push-runs.md. The workflow fix stays deferred.

base.scratch is unchanged. Its existing one-release lag behind app:latest will be addressed separately.

Compatibility

go test now enables the stdversion vet check by default. Go 1.27 can emit different gzip, zip, zlib and PNG bytes. golangci-lint 2.13.1 can report new findings from updated analyzers.

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