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/daemon/daemon.go

@@ -16,6 +16,7 @@ import (
 	"time"
 
 	"banger/internal/api"
+	"banger/internal/buildinfo"
 	"banger/internal/config"
 	"banger/internal/model"
 	"banger/internal/paths"
@@ -250,7 +251,15 @@ func (d *Daemon) dispatch(ctx context.Context, req rpc.Request) rpc.Response {
 	}
 	switch req.Method {
 	case "ping":
-		result, _ := rpc.NewResult(api.PingResult{Status: "ok", PID: d.pid, WebURL: d.webURL})
+		info := buildinfo.Current()
+		result, _ := rpc.NewResult(api.PingResult{
+			Status:  "ok",
+			PID:     d.pid,
+			WebURL:  d.webURL,
+			Version: info.Version,
+			Commit:  info.Commit,
+			BuiltAt: info.BuiltAt,
+		})
 		return result
 	case "shutdown":
 		go d.Close()