feat(protocol): add Name field to providers.Message

Adds an optional Name field on providers.Message so per-message sender
attribution can be carried through the persistence layer and onto the
wire. The field is JSON-tagged `name,omitempty`, so:

- the OpenAI Chat Completions wire format will be able to emit the
  native `name` field that disambiguates participants in multi-user
  conversations;
- session stores that round-trip messages via json.Marshal preserve
  the field with no extra plumbing;
- existing session files load cleanly with Name = "".

This is the schema-only change. Adapters and the agent pipeline are
updated in subsequent commits.

Refs #2702.

pkg/providers/protocoltypes/types.go

@@ -79,8 +79,16 @@ type Attachment struct {
 }
 
 type Message struct {
-	Role             string         `json:"role"`
+	Role    string `json:"role"`
-	Content          string         `json:"content"`
+	Content string `json:"content"`
+	// Name carries per-message sender attribution for multi-user sessions
+	// (Discord/Telegram/Slack groups, etc.). It is the sanitized SenderID
+	// from the inbound channel and is OpenAI-Chat-Completions wire-safe
+	// (matches `^[a-zA-Z0-9_-]{1,64}$`). Adapters that natively support a
+	// per-message name field (OpenAI) emit it directly; adapters that do
+	// not (Anthropic, Bedrock) prefix it onto user content at marshal
+	// time without mutating the persisted message.
+	Name             string         `json:"name,omitempty"`
 	Media            []string       `json:"media,omitempty"`
 	Attachments      []Attachment   `json:"attachments,omitempty"`
 	ReasoningContent string         `json:"reasoning_content,omitempty"`

pkg/providers/protocoltypes/types_test.go

@@ -0,0 +1,66 @@
+// PicoClaw - Ultra-lightweight personal AI agent
+// License: MIT
+//
+// Copyright (c) 2026 PicoClaw contributors
+
+package protocoltypes
+
+import (
+	"encoding/json"
+	"strings"
+	"testing"
+)
+
+// TestMessage_NameJSONRoundtrip verifies that the Name field — added for
+// multi-user sender attribution (issue #2702) — round-trips through
+// json.Marshal/Unmarshal. The session persistence layer (JSONL append +
+// in-memory snapshot via json.MarshalIndent) relies on this transparency
+// to carry sender attribution into stored history without bespoke code.
+func TestMessage_NameJSONRoundtrip(t *testing.T) {
+	in := Message{Role: "user", Content: "hi", Name: "U_alice"}
+	data, err := json.Marshal(in)
+	if err != nil {
+		t.Fatalf("json.Marshal: %v", err)
+	}
+	if !strings.Contains(string(data), `"name":"U_alice"`) {
+		t.Errorf("expected name in marshaled JSON, got: %s", string(data))
+	}
+	var out Message
+	if err := json.Unmarshal(data, &out); err != nil {
+		t.Fatalf("json.Unmarshal: %v", err)
+	}
+	if out.Name != "U_alice" {
+		t.Errorf("Name after roundtrip = %q, want U_alice", out.Name)
+	}
+}
+
+// TestMessage_NameOmittedWhenEmpty preserves wire/persistence backward
+// compatibility for direct-channel turns and pre-2702 session files: an
+// empty Name must not appear as `"name":""` in either marshaled output.
+func TestMessage_NameOmittedWhenEmpty(t *testing.T) {
+	in := Message{Role: "user", Content: "hi"}
+	data, err := json.Marshal(in)
+	if err != nil {
+		t.Fatalf("json.Marshal: %v", err)
+	}
+	if strings.Contains(string(data), `"name"`) {
+		t.Errorf("empty Name should be omitted; got: %s", string(data))
+	}
+}
+
+// TestMessage_LegacyHistoryUnmarshalsToEmptyName verifies that historical
+// session files written before this field existed still load cleanly: the
+// Name field is simply zero-valued.
+func TestMessage_LegacyHistoryUnmarshalsToEmptyName(t *testing.T) {
+	legacy := []byte(`{"role":"user","content":"old message"}`)
+	var msg Message
+	if err := json.Unmarshal(legacy, &msg); err != nil {
+		t.Fatalf("json.Unmarshal legacy: %v", err)
+	}
+	if msg.Name != "" {
+		t.Errorf("legacy Name = %q, want empty", msg.Name)
+	}
+	if msg.Content != "old message" {
+		t.Errorf("legacy Content = %q, want preserved", msg.Content)
+	}
+}