Show git commit and build time in vintner version

Uses Go's automatic VCS build-info stamping (vcs.revision/vcs.time/
vcs.modified, embedded by `go build` since Go 1.18 - no -ldflags
changes needed, works the same for a CI release build and a plain
local `go build`). Knowing the exact commit a bug report's binary was
built from, not just the X.Y.Z tag, is the point - two builds of the
same tag could still differ.

Split the pure formatting logic (formatVersion) from the
debug.ReadBuildInfo() call so it's actually unit-testable: `go test`
binaries don't get VCS stamping the way `go build` ones do, so there
was no way to exercise the revision-formatting branch through
versionString() itself.

cmd/vintner/main.go

@@ -50,7 +50,7 @@ func runCLI(args []string) int {
 	case "completion":
 		return runCompletion(args[1:])
 	case "version", "v", "--version":
-		fmt.Println("vintner " + version)
+		fmt.Println(versionString())
 		return 0
 	case "-h", "--help", "help", "h":
 		printUsage()

cmd/vintner/version.go

@@ -0,0 +1,56 @@
+package main
+
+import (
+	"fmt"
+	"runtime/debug"
+)
+
+// versionString renders "vintner <version>" plus, when available, the git
+// commit and build time Go's toolchain embeds automatically (since Go
+// 1.18, `go build` stamps vcs.revision/vcs.time/vcs.modified into the
+// binary on its own - no -ldflags needed for this part, so it works the
+// same whether the binary came from CI or a plain local `go build`).
+// Knowing the exact commit a reported bug was built from, not just the
+// X.Y.Z tag, is the point: two builds of the same tag could still differ
+// if the tag was ever moved, or if someone built from an uncommitted tree.
+//
+// Note for anyone testing this: `go build` stamps vcs.* build settings,
+// but `go test` binaries don't get them - there's no environment where a
+// `go test` run can exercise the revision-formatting branch below, hence
+// formatVersion is split out and tested directly instead.
+func versionString() string {
+	revision, buildTime, dirty := "", "", false
+	if info, ok := debug.ReadBuildInfo(); ok {
+		for _, s := range info.Settings {
+			switch s.Key {
+			case "vcs.revision":
+				revision = s.Value
+			case "vcs.time":
+				buildTime = s.Value
+			case "vcs.modified":
+				dirty = s.Value == "true"
+			}
+		}
+	}
+	return formatVersion(version, revision, buildTime, dirty)
+}
+
+// formatVersion is the pure part of versionString: given a revision (full
+// git SHA, may be empty), it's shortened to 12 chars and marked "-dirty" if
+// the build tree had uncommitted changes.
+func formatVersion(ver, revision, buildTime string, dirty bool) string {
+	v := "vintner " + ver
+	if revision == "" {
+		return v
+	}
+	if len(revision) > 12 {
+		revision = revision[:12]
+	}
+	if dirty {
+		revision += "-dirty"
+	}
+	if buildTime != "" {
+		return fmt.Sprintf("%s (%s, %s)", v, revision, buildTime)
+	}
+	return fmt.Sprintf("%s (%s)", v, revision)
+}

cmd/vintner/version_test.go

@@ -0,0 +1,46 @@
+package main
+
+import "testing"
+
+func TestFormatVersion(t *testing.T) {
+	for _, tc := range []struct {
+		name                     string
+		ver, revision, buildTime string
+		dirty                    bool
+		want                     string
+	}{
+		{
+			name: "no VCS info at all",
+			ver:  "dev",
+			want: "vintner dev",
+		},
+		{
+			name:      "clean build with full info",
+			ver:       "0.3.0",
+			revision:  "6186837fd23616335ba8aff830801692a756799c",
+			buildTime: "2026-07-25T01:33:41Z",
+			want:      "vintner 0.3.0 (6186837fd236, 2026-07-25T01:33:41Z)",
+		},
+		{
+			name:     "dirty tree",
+			ver:      "0.3.0",
+			revision: "6186837fd23616335ba8aff830801692a756799c",
+			dirty:    true,
+			want:     "vintner 0.3.0 (6186837fd236-dirty)",
+		},
+		{
+			name:     "short revision left untouched",
+			ver:      "dev",
+			revision: "abc123",
+			want:     "vintner dev (abc123)",
+		},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			got := formatVersion(tc.ver, tc.revision, tc.buildTime, tc.dirty)
+			if got != tc.want {
+				t.Errorf("formatVersion(%q, %q, %q, %v) = %q, want %q",
+					tc.ver, tc.revision, tc.buildTime, tc.dirty, got, tc.want)
+			}
+		})
+	}
+}