test(platform): 修 Windows 上平台脆弱的断言（不削弱测试意图）

- trace/task: 亚 tick 耗时合法读到 0（本机时钟步长约 0.44ms），改为断言非负且 Duration == EndedAt-StartedAt
- user/runtimeconfig: Windows 无法表示普通文件的 POSIX 权限位(Stat 恒 0666)，0600 断言仅在非 Windows 生效，注释写明实现确实请求 0600
- exec: MSYS pwd 把同一目录渲染成 /tmp/...；改用「标记文件相对名解析」+「与显式 root 的 pwd 输出逐字相等」两条跨平台断言 + 变异验证
- 只读排查全仓同类脆弱断言，结果记入 DevDocs/工作小结

internal/runtime/exec/terminal_test.go

@@ -144,20 +144,51 @@ func TestTerminal_Injection_Rejected(t *testing.T) {
 
 func TestTerminal_WorkDir_DefaultToRoot(t *testing.T) {
 	t.Parallel()
-	requireBin(t, "pwd")
-	root := t.TempDir()
-	ex := newEnabledExecutor(t, enabledCfg("pwd", root))
 
-	res, err := ex.Execute(context.Background(), ExecRequest{Command: []string{"pwd"}})
+	// Probe the default working directory without comparing rendered path
+	// strings. On Windows the command may resolve to Git's MSYS coreutils, which
+	// reports the Windows cwd it was handed as a POSIX path (the user Temp
+	// directory comes back as /tmp/...), so a "Stdout contains root" check is not
+	// portable. A marker file that exists only in the sandbox root, read through
+	// a relative name, is found only when the process really runs in that root.
+	const (
+		probeFile = "cwd-probe.txt"
+		probeBody = "cwd-probe"
+	)
+	root := t.TempDir()
+	if err := os.WriteFile(filepath.Join(root, probeFile), []byte(probeBody), 0o600); err != nil {
+		t.Fatalf("write probe: %v", err)
+	}
+	requireBin(t, "cat")
+	catEx := newEnabledExecutor(t, enabledCfg("cat", root))
+	res, err := catEx.Execute(context.Background(), ExecRequest{Command: []string{"cat", probeFile}})
 	if err != nil {
 		t.Fatalf("Execute: %v", err)
 	}
-	realRoot := root
-	if r, err := filepath.EvalSymlinks(root); err == nil {
-		realRoot = r
+	if res.ExitCode != 0 || res.Stdout != probeBody {
+		t.Errorf("cat %s with no WorkDir: exit=%d stdout=%q stderr=%q, want %q read from the sandbox root %s",
+			probeFile, res.ExitCode, res.Stdout, res.Stderr, probeBody, root)
 	}
-	if !strings.Contains(res.Stdout, realRoot) {
-		t.Errorf("Stdout = %q, want contain %q", res.Stdout, realRoot)
+
+	// And the default must be the root itself, not a parent or a subdirectory:
+	// pwd with no WorkDir must print exactly what pwd prints for WorkDir=root.
+	// Both runs share pwd's own rendering, so the comparison stays platform
+	// independent.
+	requireBin(t, "pwd")
+	pwdEx := newEnabledExecutor(t, enabledCfg("pwd", root))
+	deflt, err := pwdEx.Execute(context.Background(), ExecRequest{Command: []string{"pwd"}})
+	if err != nil {
+		t.Fatalf("Execute pwd (default workdir): %v", err)
+	}
+	explicit, err := pwdEx.Execute(context.Background(), ExecRequest{
+		Command: []string{"pwd"},
+		WorkDir: root,
+	})
+	if err != nil {
+		t.Fatalf("Execute pwd (WorkDir=%s): %v", root, err)
+	}
+	if got, want := strings.TrimSpace(deflt.Stdout), strings.TrimSpace(explicit.Stdout); got != want {
+		t.Errorf("pwd with no WorkDir = %q, want the sandbox root as pwd renders it with WorkDir=%s: %q", got, root, want)
 	}
 }