Report the Go build info version when linker flags set none (closes #422)

A `go install` of a tag sets no linker flags, so the binary reported
`dev`. Version, commit and commit date that the linker flags leave
unset now come from the build info Go records. Go records a
pseudo-version for an untagged commit and a `+dirty` suffix for a
modified checkout; `IsDevVersion` now counts both as development
builds, using `golang.org/x/mod`.

Model: opus-5-5

go.mod

@@ -24,6 +24,7 @@ require (
 	github.com/stretchr/testify v1.11.1
 	go.uber.org/dig v1.19.0
 	go.uber.org/fx v1.24.0
+	golang.org/x/mod v0.29.0
 	golang.org/x/sync v0.18.0
 	golang.org/x/sys v0.38.0
 	golang.org/x/term v0.37.0