daemon: thread per-RPC op_id end-to-end

Today there's no way to correlate a CLI failure with a daemon log
line. operationLog records relative timing but no id, two concurrent
vm.start calls log indistinguishably, and the async
vmCreateOperationState.ID is user-facing yet never reaches the
journal. The root helper logs plain text to stderr while bangerd
logs JSON, so a merged journalctl is hard to grep across the
trust-boundary split.

Mint a per-RPC op id at dispatch entry, store it on context, and
include it as an "op_id" attr on every operationLog record. The
id is stamped onto every error response (including the early
short-circuit paths bad_version and unknown_method). rpc.Call
forwards the context op id on requests so a daemon RPC and the
helper RPCs it triggers all share one id. The helper now logs
JSON to match bangerd, adopts the inbound id, and emits a single
"helper rpc completed" / "helper rpc failed" line per call so
operators can see at a glance how long each privileged op took.

vmCreateOperationState.ID is now the same id dispatch generated
for vm.create.begin — one identifier between client status polls,
daemon logs, and helper logs.

The wire format gains two optional fields: rpc.Request.OpID and
rpc.ErrorResponse.OpID, both omitempty so older peers (and the
opposite direction) ignore them. ErrorResponse.Error() now appends
"(op-XXXXXX)" to its string form when set; existing callers that
just print err.Error() get the id for free.

Tests cover: dispatch stamps op_id on unknown_method, bad_version,
and handler-returned errors; rpc.Call exposes the typed
*ErrorResponse via errors.As so the CLI can read code/op_id; ctx
op_id is forwarded to the server in the request envelope.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/rpc/rpc.go

@@ -18,6 +18,40 @@ type Request struct {
 	Version int             `json:"version"`
 	Method  string          `json:"method"`
 	Params  json.RawMessage `json:"params,omitempty"`
+	// OpID is the per-RPC correlation id. Optional on the wire so
+	// older clients (which don't set it) and older servers (which
+	// don't read it) keep interoperating. The daemon attaches it on
+	// every incoming request via dispatch; rpc.Call forwards
+	// whatever id is on ctx so a helper RPC carries the same id as
+	// the daemon RPC that triggered it.
+	OpID string `json:"op_id,omitempty"`
+}
+
+// opIDKey is the context-value key for the per-RPC correlation id
+// that flows from CLI → daemon → root helper. Lives in the rpc
+// package because rpc.Call needs to read it without depending on
+// the daemon package; daemon and roothelper both import it.
+type opIDKey struct{}
+
+// WithOpID stores opID on ctx. Used by the daemon dispatch layer to
+// inject the per-request id; rpc.Call picks it up automatically.
+func WithOpID(ctx context.Context, opID string) context.Context {
+	if ctx == nil || opID == "" {
+		return ctx
+	}
+	return context.WithValue(ctx, opIDKey{}, opID)
+}
+
+// OpIDFromContext returns the op id stored on ctx by WithOpID, or
+// "" if none was set.
+func OpIDFromContext(ctx context.Context) string {
+	if ctx == nil {
+		return ""
+	}
+	if id, _ := ctx.Value(opIDKey{}).(string); id != "" {
+		return id
+	}
+	return ""
 }
 
 type Response struct {
@@ -29,6 +63,29 @@ type Response struct {
 type ErrorResponse struct {
 	Code    string `json:"code"`
 	Message string `json:"message"`
+	// OpID is the daemon-assigned correlation id for the RPC that
+	// produced this error. Optional and may be empty (older daemons
+	// don't set it); when present the CLI surfaces it so an operator
+	// can grep journalctl by that id and find the full context.
+	OpID string `json:"op_id,omitempty"`
+}
+
+// Error makes ErrorResponse satisfy the error interface so callers
+// can errors.As it out of an rpc.Call return value and read the
+// structured fields directly. The default string form is
+// "code: message (op-id)" — the op id only appears when the daemon
+// attached one. CLI code paths that want a translated, user-facing
+// message render the typed fields themselves; this fallback is for
+// log lines, fmt.Errorf %w wrappers, and any caller that hasn't
+// bothered to errors.As yet.
+func (e *ErrorResponse) Error() string {
+	if e == nil {
+		return ""
+	}
+	if e.OpID == "" {
+		return e.Code + ": " + e.Message
+	}
+	return e.Code + ": " + e.Message + " (" + e.OpID + ")"
 }
 
 func NewResult(v any) (Response, error) {
@@ -43,6 +100,12 @@ func NewError(code, message string) Response {
 	return Response{OK: false, Error: &ErrorResponse{Code: code, Message: message}}
 }
 
+// NewErrorWithOpID is the variant for daemon dispatch sites that have
+// resolved an op id by the time they encode the response.
+func NewErrorWithOpID(code, message, opID string) Response {
+	return Response{OK: false, Error: &ErrorResponse{Code: code, Message: message, OpID: opID}}
+}
+
 func DecodeParams[T any](req Request) (T, error) {
 	var zero T
 	if len(req.Params) == 0 {
@@ -78,7 +141,7 @@ func Call[T any](ctx context.Context, socketPath, method string, params any) (T,
 		_ = conn.SetDeadline(deadline)
 	}
 
-	request := Request{Version: Version, Method: method}
+	request := Request{Version: Version, Method: method, OpID: OpIDFromContext(ctx)}
 	if params != nil {
 		raw, err := json.Marshal(params)
 		if err != nil {
@@ -105,7 +168,10 @@ func Call[T any](ctx context.Context, socketPath, method string, params any) (T,
 		if response.Error == nil {
 			return zero, errors.New("rpc error")
 		}
-		return zero, fmt.Errorf("%s: %s", response.Error.Code, response.Error.Message)
+		// Return the typed error directly so callers that need code
+		// or op_id can errors.As it out. err.Error() format is
+		// preserved for callers that only print the message.
+		return zero, response.Error
 	}
 	if len(response.Result) == 0 {
 		return zero, nil