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
+}