feat(messageutil): add SanitizeMessageName for OpenAI wire compliance

OpenAI's Chat Completions name field requires ^[a-zA-Z0-9_-]{1,64}$.
Real-world sender IDs (Discord alice#1234, numeric Telegram IDs, Slack
U07AB12C3DEF, free-form CLI names) often violate this, so a sanitizer
sits between the raw inbound ID and any wire payload.

Disallowed characters collapse into a single underscore, leading and
trailing underscores are trimmed, and the result is byte-safely
truncated to 64 (output is pure ASCII).

This helper will be wired into the agent pipeline in a later commit.

Refs #2702.

pkg/providers/messageutil/messageutil.go

@@ -1,11 +1,43 @@
 package messageutil
 
 import (
+	"regexp"
 	"strings"
 
 	"github.com/sipeed/picoclaw/pkg/providers/protocoltypes"
 )
 
+// nameSanitizer matches characters that are not valid in the OpenAI Chat
+// Completions `name` field. The OpenAI spec requires `^[a-zA-Z0-9_-]{1,64}$`.
+var nameSanitizer = regexp.MustCompile(`[^a-zA-Z0-9_-]+`)
+
+// maxMessageNameLen mirrors the OpenAI Chat Completions hard limit on the
+// `name` field. Values longer than this are truncated.
+const maxMessageNameLen = 64
+
+// SanitizeMessageName normalizes a raw sender identifier into a value safe
+// for the OpenAI Chat Completions `name` field and stable across providers.
+// Disallowed characters are coalesced into a single underscore; leading and
+// trailing underscores are trimmed; the result is truncated to 64 bytes.
+//
+// Returns "" when raw is empty or collapses to nothing after sanitization.
+// The output is pure ASCII so the truncation is byte-safe.
+func SanitizeMessageName(raw string) string {
+	raw = strings.TrimSpace(raw)
+	if raw == "" {
+		return ""
+	}
+	cleaned := nameSanitizer.ReplaceAllString(raw, "_")
+	cleaned = strings.Trim(cleaned, "_")
+	if cleaned == "" {
+		return ""
+	}
+	if len(cleaned) > maxMessageNameLen {
+		cleaned = cleaned[:maxMessageNameLen]
+	}
+	return cleaned
+}
+
 // IsTransientAssistantThoughtMessage reports whether msg is an invalid
 // reasoning-only assistant history record. These "hanging" thought messages
 // are not a canonical persisted format and should be discarded instead of

pkg/providers/messageutil/messageutil_test.go

@@ -0,0 +1,67 @@
+// PicoClaw - Ultra-lightweight personal AI agent
+// License: MIT
+//
+// Copyright (c) 2026 PicoClaw contributors
+
+package messageutil
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestSanitizeMessageName(t *testing.T) {
+	tests := []struct {
+		name string
+		in   string
+		want string
+	}{
+		{"empty", "", ""},
+		{"whitespace only", "   ", ""},
+		{"plain ascii", "alice", "alice"},
+		{"discord-style id", "alice#1234", "alice_1234"},
+		{"telegram numeric id", "141455495", "141455495"},
+		{"slack-style id", "U07AB12C3DEF", "U07AB12C3DEF"},
+		{"already valid mixed case", "Alice_Bob-99", "Alice_Bob-99"},
+		{"strips leading/trailing underscores", "@alice@", "alice"},
+		{"collapses runs", "a@@@b", "a_b"},
+		{"non-ascii becomes underscores", "李华", ""},
+		{"non-ascii mixed", "alice 李", "alice"},
+		{"all special chars collapses to empty", "@@@!!!", ""},
+		{"truncates to 64", strings.Repeat("a", 100), strings.Repeat("a", 64)},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := SanitizeMessageName(tt.in)
+			if got != tt.want {
+				t.Errorf("SanitizeMessageName(%q) = %q, want %q", tt.in, got, tt.want)
+			}
+		})
+	}
+}
+
+func TestSanitizeMessageName_OutputAlwaysWireSafe(t *testing.T) {
+	inputs := []string{
+		"alice#1234", "U07AB12C3DEF", "141455495",
+		"@@@", "李华", "alice 李", "Alice_Bob-99",
+		strings.Repeat("a", 200),
+	}
+	for _, in := range inputs {
+		got := SanitizeMessageName(in)
+		if got == "" {
+			continue
+		}
+		if len(got) > maxMessageNameLen {
+			t.Errorf("SanitizeMessageName(%q) length %d exceeds max %d", in, len(got), maxMessageNameLen)
+		}
+		for _, r := range got {
+			ok := (r >= 'a' && r <= 'z') ||
+				(r >= 'A' && r <= 'Z') ||
+				(r >= '0' && r <= '9') ||
+				r == '_' || r == '-'
+			if !ok {
+				t.Errorf("SanitizeMessageName(%q) = %q contains invalid rune %q", in, got, r)
+			}
+		}
+	}
+}