Stamp shared build metadata into banger binaries

Treat `banger`, `bangerd`, and `banger-vsock-agent` as one release by
stamping the same version, commit SHA, and build timestamp into every
binary through a shared ldflag-backed `internal/buildinfo` package.

Add `banger version`, extend daemon ping/status to report the running
daemon's build tuple, and keep the guest helper linked to the same build
metadata without adding a new public version surface for it.

Validate with `GOCACHE=/tmp/banger-gocache go test ./...`, `make build`,
`./build/bin/banger version`, and `./build/bin/banger daemon status`
after the daemon restarts onto the new binary.

internal/buildinfo/buildinfo.go

@@ -0,0 +1,34 @@
+package buildinfo
+
+import "strings"
+
+var (
+	Version = "dev"
+	Commit  = "unknown"
+	BuiltAt = "unknown"
+)
+
+type Info struct {
+	Version string
+	Commit  string
+	BuiltAt string
+}
+
+func Current() Info {
+	return Normalize(Version, Commit, BuiltAt)
+}
+
+func Normalize(version, commit, builtAt string) Info {
+	return Info{
+		Version: normalizedValue(version, "dev"),
+		Commit:  normalizedValue(commit, "unknown"),
+		BuiltAt: normalizedValue(builtAt, "unknown"),
+	}
+}
+
+func normalizedValue(value, fallback string) string {
+	if trimmed := strings.TrimSpace(value); trimmed != "" {
+		return trimmed
+	}
+	return fallback
+}

internal/buildinfo/buildinfo_test.go

@@ -0,0 +1,33 @@
+package buildinfo
+
+import "testing"
+
+func TestNormalizeUsesFallbacks(t *testing.T) {
+	t.Parallel()
+
+	info := Normalize("", "  ", "\t")
+	if info.Version != "dev" {
+		t.Fatalf("Version = %q, want dev", info.Version)
+	}
+	if info.Commit != "unknown" {
+		t.Fatalf("Commit = %q, want unknown", info.Commit)
+	}
+	if info.BuiltAt != "unknown" {
+		t.Fatalf("BuiltAt = %q, want unknown", info.BuiltAt)
+	}
+}
+
+func TestNormalizeTrimsValues(t *testing.T) {
+	t.Parallel()
+
+	info := Normalize(" v1.2.3 ", " abc123 ", " 2026-03-22T12:00:00Z ")
+	if info.Version != "v1.2.3" {
+		t.Fatalf("Version = %q, want v1.2.3", info.Version)
+	}
+	if info.Commit != "abc123" {
+		t.Fatalf("Commit = %q, want abc123", info.Commit)
+	}
+	if info.BuiltAt != "2026-03-22T12:00:00Z" {
+		t.Fatalf("BuiltAt = %q, want 2026-03-22T12:00:00Z", info.BuiltAt)
+	}
+}