fix(weixin): probe sync dir writability before choosing home

pkg/channels/weixin/state.go

@@ -36,12 +36,53 @@ type syncCursorFile struct {
 	GetUpdatesBuf string `json:"get_updates_buf"`
 }
 
+func syncDirForHome(home string) string {
+	return filepath.Join(home, "channels", "weixin", "sync")
+}
+
+func probeSyncDirWritable(home string) error {
+	syncDir := syncDirForHome(home)
+	if err := os.MkdirAll(syncDir, 0o700); err != nil {
+		return err
+	}
+
+	probeFile, err := os.CreateTemp(syncDir, ".write-probe-*")
+	if err != nil {
+		return err
+	}
+	probePath := probeFile.Name()
+	if err := probeFile.Close(); err != nil {
+		_ = os.Remove(probePath)
+		return err
+	}
+	if err := os.Remove(probePath); err != nil {
+		return err
+	}
+	return nil
+}
+
 func picoclawHomeDir() string {
 	if home := os.Getenv(config.EnvHome); home != "" {
 		return home
 	}
-	userHome, _ := os.UserHomeDir()
-	return filepath.Join(userHome, ".picoclaw")
+
+	if userHome, err := os.UserHomeDir(); err == nil && userHome != "" {
+		home := filepath.Join(userHome, ".picoclaw")
+		if probeErr := probeSyncDirWritable(home); probeErr == nil {
+			return home
+		} else {
+			logger.WarnCF(
+				"weixin",
+				"Default picoclaw home is not writable; using temp directory for sync cursor",
+				map[string]any{
+					"path":  home,
+					"error": probeErr.Error(),
+				},
+			)
+		}
+	}
+
+	return filepath.Join(os.TempDir(), "picoclaw")
 }
 
 func buildWeixinSyncBufPath(cfg config.WeixinConfig) string {

pkg/channels/weixin/weixin_test.go

@@ -7,7 +7,9 @@ import (
 	"errors"
 	"io"
 	"net/http"
+	"os"
 	"path/filepath"
+	"runtime"
 	"testing"
 	"time"
 
@@ -159,6 +161,67 @@ func TestBuildWeixinSyncBufPathUsesPicoclawHome(t *testing.T) {
 	}
 }
 
+func TestBuildWeixinSyncBufPathFallsBackWhenHomeIsUnusable(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.Skip("HOME handling differs on windows")
+	}
+
+	t.Setenv(config.EnvHome, "")
+	unusableHome := filepath.Join(t.TempDir(), "home-file")
+	if err := os.WriteFile(unusableHome, []byte("not-a-dir"), 0o600); err != nil {
+		t.Fatalf("WriteFile(unusableHome) error = %v", err)
+	}
+	t.Setenv("HOME", unusableHome)
+
+	wxCfg := config.WeixinConfig{
+		BaseURL: "https://ilinkai.weixin.qq.com/",
+	}
+	wxCfg.SetToken("token-123")
+	got := buildWeixinSyncBufPath(wxCfg)
+	wantDir := filepath.Join(os.TempDir(), "picoclaw", "channels", "weixin", "sync")
+	if filepath.Dir(got) != wantDir {
+		t.Fatalf("sync path dir = %q, want %q", filepath.Dir(got), wantDir)
+	}
+}
+
+func TestBuildWeixinSyncBufPathFallsBackWhenSyncDirUnwritable(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.Skip("permission mode behavior differs on windows")
+	}
+	if os.Geteuid() == 0 {
+		t.Skip("root can usually bypass directory write permissions")
+	}
+
+	t.Setenv(config.EnvHome, "")
+	home := t.TempDir()
+	t.Setenv("HOME", home)
+
+	syncDir := filepath.Join(home, ".picoclaw", "channels", "weixin", "sync")
+	if err := os.MkdirAll(syncDir, 0o700); err != nil {
+		t.Fatalf("MkdirAll(syncDir) error = %v", err)
+	}
+	if err := os.Chmod(syncDir, 0o500); err != nil {
+		t.Fatalf("Chmod(syncDir, 0500) error = %v", err)
+	}
+	t.Cleanup(func() { _ = os.Chmod(syncDir, 0o700) })
+
+	if probeFile, err := os.CreateTemp(syncDir, ".probe-*"); err == nil {
+		_ = probeFile.Close()
+		_ = os.Remove(probeFile.Name())
+		t.Skip("environment does not enforce non-writable directory permissions")
+	}
+
+	wxCfg := config.WeixinConfig{
+		BaseURL: "https://ilinkai.weixin.qq.com/",
+	}
+	wxCfg.SetToken("token-123")
+	got := buildWeixinSyncBufPath(wxCfg)
+	wantDir := filepath.Join(os.TempDir(), "picoclaw", "channels", "weixin", "sync")
+	if filepath.Dir(got) != wantDir {
+		t.Fatalf("sync path dir = %q, want %q", filepath.Dir(got), wantDir)
+	}
+}
+
 func TestSessionPauseGuard(t *testing.T) {
 	ch := &WeixinChannel{
 		typingCache: make(map[string]typingTicketCacheEntry),