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

internal/globals/globals_test.go

@@ -1,6 +1,7 @@
 package globals_test
 
 import (
+	"runtime/debug"
 	"testing"
 
 	"sneak.berlin/go/vaultik/internal/globals"
@@ -59,12 +60,20 @@ func TestIsDevVersion(t *testing.T) {
 		{"1.0.0-rc.1-12-g877eb2f", true},
 		// A tagged commit with uncommitted changes is not that tag.
 		{"v1.0.0-dirty", true},
+		// What Go records for a build without linker flags: a
+		// pseudo-version for an untagged commit, "+dirty" for a
+		// modified checkout.
+		{"v1.0.0-rc3.0.20261111001226-abcdefabcdef", true},
+		{"v0.0.0-20261111001226-abcdefabcdef", true},
+		{"v1.0.1-0.20261111001226-abcdefabcdef", true},
+		{"v1.0.0+dirty", true},
 		// What a tagged build produces (goreleaser's .Version strips
 		// the leading "v"; script/version keeps it).
 		{"1.0.0", false},
 		{"0.1.0", false},
 		{"1.0.0-rc.1", false},
 		{"v1.0.0", false},
+		{"v1.0.0-rc3", false},
 		// A release must not be mistaken for a dev build just because
 		// the string happens to contain "dev".
 		{"1.0.0-dev", false},
@@ -86,10 +95,9 @@ func TestIsDevVersion(t *testing.T) {
 	}
 }
 
-// TestDefaultVersionIsDev pins the linker-flag contract: an unstamped
-// binary (no -ldflags at all, which is what `go build ./...` and `go
-// install` produce) must report itself as a development build rather
-// than as some default release number.
+// TestDefaultVersionIsDev pins the default: a binary whose linker flags
+// and build info name no version must report itself as a development
+// build rather than as some default release number.
 func TestDefaultVersionIsDev(t *testing.T) {
 	t.Parallel()
 
@@ -98,3 +106,104 @@ func TestDefaultVersionIsDev(t *testing.T) {
 			globals.DevVersion)
 	}
 }
+
+// buildInfo returns build info with the given main module version and,
+// when revision is not empty, the commit settings Go records for a
+// build from a git checkout.
+func buildInfo(version, revision string) *debug.BuildInfo {
+	info := &debug.BuildInfo{
+		Main: debug.Module{Path: "sneak.berlin/go/vaultik", Version: version},
+	}
+
+	if revision != "" {
+		info.Settings = []debug.BuildSetting{
+			{Key: "vcs.revision", Value: revision},
+			{Key: "vcs.time", Value: "2026-11-10T23:12:26Z"},
+		}
+	}
+
+	return info
+}
+
+// TestFromBuildInfo covers a binary from `go install` of a tag, which
+// carries no linker flags: it must report the tag. Values the linker
+// flags set are kept.
+func TestFromBuildInfo(t *testing.T) {
+	t.Parallel()
+
+	const (
+		tag = "v1.0.0-rc3"
+		rev = "0123456789abcdef0123456789abcdef01234567"
+	)
+
+	cases := []struct {
+		name                    string
+		info                    *debug.BuildInfo
+		version, commit, date   string
+		wantVersion, wantCommit string
+		wantDate                string
+	}{
+		{
+			name:        "go install of a tag",
+			info:        buildInfo(tag, ""),
+			version:     globals.DevVersion,
+			commit:      globals.Unknown,
+			date:        globals.Unknown,
+			wantVersion: tag,
+			wantCommit:  globals.Unknown,
+			wantDate:    globals.Unknown,
+		},
+		{
+			name:        "empty version",
+			info:        buildInfo(tag, ""),
+			version:     "",
+			commit:      globals.Unknown,
+			date:        globals.Unknown,
+			wantVersion: tag,
+			wantCommit:  globals.Unknown,
+			wantDate:    globals.Unknown,
+		},
+		{
+			name:        "go build in a git checkout",
+			info:        buildInfo(tag, rev),
+			version:     globals.DevVersion,
+			commit:      globals.Unknown,
+			date:        globals.Unknown,
+			wantVersion: tag,
+			wantCommit:  rev,
+			wantDate:    "2026-11-10",
+		},
+		{
+			name:        "Go could not tell the version",
+			info:        buildInfo("(devel)", ""),
+			version:     globals.DevVersion,
+			commit:      globals.Unknown,
+			date:        globals.Unknown,
+			wantVersion: globals.DevVersion,
+			wantCommit:  globals.Unknown,
+			wantDate:    globals.Unknown,
+		},
+		{
+			name:        "linker flags set",
+			info:        buildInfo(tag, rev),
+			version:     "v1.0.0-rc2-4-gabcdef1",
+			commit:      "abcdef1",
+			date:        "2026-11-01",
+			wantVersion: "v1.0.0-rc2-4-gabcdef1",
+			wantCommit:  "abcdef1",
+			wantDate:    "2026-11-01",
+		},
+	}
+
+	for _, tc := range cases {
+		version, commit, date := globals.FromBuildInfo(
+			tc.info, tc.version, tc.commit, tc.date)
+
+		if version != tc.wantVersion || commit != tc.wantCommit ||
+			date != tc.wantDate {
+			t.Errorf("%s: got %q, %q, %q; want %q, %q, %q", tc.name,
+				version, commit, date,
+				tc.wantVersion, tc.wantCommit, tc.wantDate)
+		}
+	}
+}